Well, I guess I'll start this one...
Brian, searching for a used Deckel is not unlike searching for any other vertical mill. There are the standard checks to make regarding wear and abuse.
Most of the experience I have is with FP-1's, so I'll suggest a couple things on this model...
First, lets talk taper... manual Deckels use an odd MT4 or ISO 40 tooling. Both types of tooling can be hard, or expensive to get, so how much tooling the machine comes with is an issue. The original tooling for Deckels is perhaps superior in design to many because the male threaded ends allow for a "pass-through-the-adapter" approach to making the tooling more flexible. I don't have a good way to explain that, but it was a well thought out system. The problem is that there is not too much of it available on the used market.
Now, if you wish to trade off to a more standard tooling size, the ISO 40 machines can be converted to NMTB40 with nothing more than a draw bar that you could even make from a bolt. The MT4 machines can be adapted to R8 with a simple sleeve/drawbar combo. Keep in mind though, that you have 3 spindles to deal with...the vertical, the horzontal, and the less talked about indexer "spindle" if you own the factory indexing table attachment. So doing a conversion could entail up to 6 pieces that you would have to make. I've made these before and plan to offer them for sale soon, but I'm waiting on a good ID/OD grinder to continue the project. I have not seen any others offered for sale, but the FP-1/Lathes UK site mentions such an adapter so it's possible they exist somewhere else.
Regardless, I suggest either a Deckel with a nice compliment of tooling, or doing a conversion (which is the road I am taking even though I've had a very complete tooling set).
Next, if you are standing in front of a Deckel with the spindle going, you are going to hear noise that could scare you into thinking the bearings are bad. Most often this is the normal sound of the geartrain at work. Aside from a V-belt which links the motor to the geartrain, the whole machine is like a big watch, with all sorts of gears going on...even in the heads.
By nature these gears whir and rattle a bit.
It is possible that one of these machines could have a bad bearing, but more likely from the perspective of someone who is used to a belt driven machine, that it is the normal sound. You would either have to be familiar with the sound, or do some very close audio inspection of the machine to differenciate between the two.
Another thing you may come across is powerfeeds that don't work. Especially if the machine has been touched or demo'd by a "non-Deckelite". The machines have a built-in safeguard against overtravel or very forceful resistance to the travels. This is a shear pin which is "hidden" in the bottom of the machine (but still very accessible), which will break if the travels are overloaded. This takes about 2 minutes to repair, but can give one the impression that the powerfeeds don't work.
In normal operation, you set the limits on the travels so this doesn't happen by the placement of "travel-stops" which activate a linkage which physically pushes that big lever out of place when they are contacted.
For some reason though, it is common to find these stops missing. They are easy to make though if your potiential machine is lacking them.
Something else about the powerfeeds... They only work with the machine's motor turning in the "correct" direction (which is indicated by an arrow on the back of the machine). If you plug one in to test and the powerfeed lever doesn't want to go into gear, check this first. The machine will run in both directions, but the powerfeeds will only work in one. I have yet to need reverse spindle rotation in my milling, but when it does eventually happen, I will have to suffer through manually turning the dials...and so will you.
Another thing I am learning is pretty common is to have the machines filled with grease. In some years, Deckel fitted the oil channels with those high-pressure grease fittings, which I understand were supposed to be used with high-pressure oil guns but would also accept grease guns.
Somewhere along the line, someone lost the manual that states "oil" and the logical thing for the casual onlooker to do was to use what fits...the grease gun.
I have a friend who is taking half of his machine apart to remove the grease from one of his machines. As far as I know, Deckel is not the only brand of machine that suffers from this odd cross-over. I haven't heard of any actual damage being done by the grease yet, but I think the majority would agree that grease is not the best thing to have in a machine tool (unless it's in a sealed place).
The ways on a Deckel style machine are far better protected from chips, dust, and other contaminates than the standard knee mill. This is simply a matter of design since there are no exposed horizontal ways as you would find on the Y axis of a Bridgeport style machine. For this, the ways of these machines are usually in pretty good shape.
Another thing to take into consideration is paint. It is easy to determine a Deckel being in fantastic shape by partially considering the condition of the paint, but unlike most mills, Deckels also seem to well outlast their paint by a far margin so it becomes difficult to say that one with bad paint, or a really bad dealer paint-over is worn. Even though the the quality of the machine is very high, they were still limited to the paint technology of the time...which was not that good.
Anyway, a beat "looking" Deckel could still be a very well maintained and close to perfect machine, so keep this in mind.
Dials....
At least on the FP-1, either several dials were available, or they changed the default dial from time to time. These range from metric, to .001/.100 to .001/.125 (an eighth) and even one more I think.
Personally, I use DRO's or indicators exlusively so this doesn't bother me a bit, but if you plan on using the dials, then you are going to need to consider this. I wouldn't pass up any nice Deckel based on this, but if you really want to be picky...well...
Backlash...
Most Deckels I've come across have a considerable amount of perceived backlash. A great deal of this is in how the lead screws are attached to the working table. They use some sort of thrust washer/bearing/nut arrangement which will allow the screw to move "through" the table a bit before it connects and starts moving the table itself. For this, it can be very hard to tell what is leadscrew wear vs a mal-adjusted "thrust nut". More often than not, it is the nut, or at least mostly the nut. I haven't gotten very far into this subject as again, if I care, I'll use a DRO or indicator. Deckels have quite a bit of accomidations for indicators...the placements are built into the machine, but I suppose you could easily fit a Mitutoyo style digital scale unit.
Motors...
Although the motors came in a variety of voltages, they apparently are easy to swap out with whatever you would like, and from looking at the arrangement, this makes sense. Many though are 460v, so if you want to start making chips that same day, you will have to consider this. If one of my motors ever dies though, I'll just replace it with a Baldor "E" class.
Speaking of electrics... the later FP-1's (and others) have a separate external "power box" with mag starters and other OSHA style requirements. This is both good and bad. If you have room for this, you can set your tooling on top of it, and have a bit of extra safety. However, the older machines are wired direct, which makes for a smaller footprint, and also makes them easier to move...etc, but they WILL start if the switch is on when the power is restored. The older models don't have an E-stop button either, which is a related topic.
I don't have any personal preference on this, but I will say that this "OSHA" box can be a PITA. Then again, maybe it will save my butt someday.
Personally, I feel that Deckels are very robust. I've had about 10 pass through here so far and havent found one that didn't work yet. The most "abuse" I've seen is lots of tool marks, grooves, and holes in the table surface.
I've been lucky enough to aquire some extremely nice FP-1's for my personal use through being in the right place at the right time and doing some trading up as well as some thrifty eating for a month or two, but I must say that I would be happy and proud to own every one that's come through here so far. I've yet to see one that wouldn't serve it's owner well. Of course, if you are of my personality, once you get into it, it is possible that you will start seeking "perfection" and it's not unlike classic car owners who go "oh..wait, this is not the stock rear tailight lense screw". I suppose productive people wont have to deal with that train of thought, but it can be fun too.
I hope this helps to get you started.
Sean