You need at least a 16" for domestic drums etc. but a 12" will do if you stay with imports. Make sure you get a quick change gear box and that bed wear is not excessive if you intend to turn parts directly to size. I would bring along a 0.0005" reading dial indicator and check the spindle runout, use a bar inserted into the spindle to load it one way and then the other...anything more than 0.001" and I would look elsewhere if it's a roller bearing headstock. Plain bearing headstocks can be refurbed easily by removing shims typically.
Spindle bore may be important to you if you intend to shorten driveshafts etc. the bigger the better. Try and get all the accessories like steady rests and taper attachements, these are very hard to find items for old lathes and getting them used may be next to impossible. A turret tool post (or better yet a cam lock)is a must, those old single adjustable tool holders are a pain to work with and not very rigid.
Don't forget horsepower either, two horse is about bare minimum for metal. Good luck hunting.