The combo machines don't have the size or accuracy for gun work. They are probably fine for hobby work, making odds and ends, etc., but every time you turn around, you won't be able to do something you need to. A 13 x 40 is a good size for gun work. A large spindle bore is good (1 1/2" at least), a slow spindle speed (40 rpm or so), is good. Anything higher than 1000 rpm scares the crap out of me. You need a steady rest and a follow rest and a good quickchange tool holder (and a lot of tooling). If you don't have 3 phase power, a single phase motor is good. I use a 3-5 hp static converter, but it's not ideal. I like the D-1 spindle nose better than a threaded nose, mostly because it won't spin a chuck off when you run in reverse. I also like the knob and lever threading quickchange better than the caliper type. Less chance of screwing up thread pitch.
Maybe someone here can advise you on how to evaluate a lathe's accuracy before you buy it, because all the rest is useless if it won't hold tolerance turning, facing, and threading.
I got too long-winded, but a good lathe is irreplaceable, and a bad one is un-usable.
Good luck, Tom
Texas State Rifle Association Life Member
NRA Endowment Life Member
A big fast bullet will beat a little fast bullet every time