Like most things, it depends on exactly what you want to do. I've got a Grizzly model 4003 12" x 36" geared-head lathe that I use to cut chambers, crown and thread barrels and make various items of tooling. So far I've worked entirely on 98 Mausers. I'm happy with the lathe. If I take my time it is capable of very accurate work.
The bed is long enough to accommodate the biggest barrel blank I expect to ever work on and complete barreled actions. The spindle bore is threaded so I can install a "spider" and big enough to slide the chamber end of the barrel through. There are plenty of speeds and thread-cutting options available, although I wish for a slower speed when cutting chambers. The other drawback is the lack of a taper attachment, so I buy barrels pre-contoured rather then contour them myself.
Roy Dunlap, in his book "Gunsmithing" recommends at least a 1.25" hole in the tailstock, a minimum of 36" bed length, both 3- and 4-jaw chucks, and as many threading gears as you can possibly get due to the many unusual threads encountered in gunsmithing work (especially of older or non-US guns). Note that a large swing is NOT needed for gunsmithing work.
Another popular line of lathes priced for the amateur is Jet. I know gunsmiths here in Minnesota who have Grizzly or Jet lathes and are pleased with them.
Used lathes from manufacturers like South Bend, Logan, Atlas and Clausing seem to be widely available. When I was looking a few years back I found that the cheap ones had been ridden hard and put away wet, while the lathes that had been well taken care of and didn't show obvious signs of excessive wear were more expensive than buying a brand new imported Taiwan-made lathe from Grizzly or Enco. If you are going to buy used, I recommend you make friends with an experienced machinist and look very carefullly before buying.
I read Dunlap's book and Steve Acker's "The Gunsmith Machinist" before I bought my lathe and found both books to be very helpful.
Good luck!
