I am unfamiliar with the weight/size of the lathe the OP wants the bench for. I've used cast iron industrial sewing machine legs for a couple of work-benches. These are light-weight legs, and were made to be be assembled in any number of units (mounting a row of industrial sewing machine heads with a common top surface). I found two sets of legs by the same manufacturer in the streets, set out for junk pickup. One set in Brooklyn in the 60's, one set in Westchester County in the 80's.
These legs are on the light side, and rely on tie-rods on the bottom (which also served as a foot clutch treadle pivot), and a cast iron panel which the user's knees would be pointing at.
I've seen heavier duty industrial sewing machine legs made of pressed steel. These legs had no aesthetics to them, simple utilitarian design.
The last set of cast iron sewing machine legs went to my nephew as part of a jeweler's work bench. Custom wood top with 'scooped area' to enable a person to sit up close to the bench, drawer for tools, and high back with cubbys, shelves and pegboard for tools. I also added a bracket and hook for a Foredom flex shaft. It made a great bench and my nephew uses it in his apartment. That being said, I'd be hesitant to use those same legs for something like a small metalworking lathe, thinking they were a bit too light.
My own method of addressing needs for a number of benches and stands is to make them myself. I fabricate what's needed using steel and welded fabrication. Using light structural steel (angle, hollow square tube, flat bar, pipe), I've made quite a few benches and stands. I make what I call "bench bucks"- fabricated steel frames which have two legs and horizontal members to support the top and shelves. The beauty of designing/fabricating bench bucks and similar is I can build exactly what's needed in terms of dimensions, as well as features (such as framing members to mount a motor and jackshaft). I often include 'foot pad plates' on the bottoms of each leg and drill them for levelling/vibration isolator pads. I am also spoiled by having friends with sawmills. I use full-dimension rough sawn lumber for some projects. Getting rough sawn is a lot cheaper than going to a big-box homeowner's type store. One friend has a drying shed and planer, and also saws some hardwood. I get rough sawn 2" x 12" planks and have them planed for top material. I run the planks thru my jointer and dowel/edge glue them for the tops. I glue/screw battens to the underside of the tops and this makes a really solid top. The tops get carriage bolted to the bench bucks, and the result is a solid bench that does not sway or wobble.
Trying to find an old workbench or something adaptable (such as an old heavy desk) could wind up taking a long time. Another style of desk is made of pressed steel and is sometimes known as a 'tanker desk'. These were popular into maybe the 60's. For a time, these tanker desks were junked, but now have been 'discovered'. A tanker desk will have a sheet-metal top surfaced with a composite matting (sort of like floor covering). A tanker desk could be given a top made of 3/4" plywood. I'd use carriage bolts and fender washers to mount the plywood on top of the tanker desk. Glue and screw on a second course of plywood if hiding the carriage bolts is desired. A couple of coats of finish to seal the plywood against oil and the tanker desk could become a good lathe bench. The plywood top would allow mounting of a jackshaft and motor. A lip could be glued/screwed to three sides of the top to contain tools and chips. I've used scrap 3/4 concrete form plywood for this sort of project. 3/4" form plywood (at least 35 years ago when I was working on hydroelectric construction project sites) was a lot better plywood than the CDX stuff. Scrap from making concrete forms was often tossed into a skip box for disposal, so the price was right.