Now you need a cincinnati #2 or Norton Tool and cutter grinder to keep your cutters in shape.
Since you have a shaper, don't spend a lot on slab mills.
Plus they would not be the best match for a small horizontal anyway.
But any other kind of horizontal cutters are good.
Saws or all thickness, slotting cutters of all thickness and configuration, angle and double angle cutters, and *especially* gear cutters are a good match for horizontals.
You probably won't use them often if at all, but round (cove cutting) and corner rounding cutters sometimes go cheap at auctions so you might as well pick them up if seen.
I just picked up a Whitney #6 (slightly bigger travels than yours, plus rise and fall head) for $5 at auction.
The good is that it is all rescraped by the PO who aged out. The bad is that mine does not have the correct spindle (seems to have a trashed grinding spindle. Someone previous to the immediate PO must have been using it as a tool and cutter grinder). No motor nor any other tooling.
If you are getting a vertical head and a slotter with yours, besides the overarm, vise, arbors, and assorted tooling, that is quite useful. Slotters will solve a lot of otherwise difficult or expensive tasks. Even if only internal keyways (without broaches). With a rotary table you can do all sorts of internal splines and gears.
smt