Try to get one with the chuck included.  There are several different standards for attaching the chuck to the lathe, and if you buy the chucks separately you will have to make sure they fit your machine.  Often the chucks, centers, and tool holders can cost more than the basic machine.  The more you get at once the better chance you have of getting a decent value.  If you expect to do a lot of thread work, be aware that most older American machines will not cut metric threads.  A taper attachment is worth a lot, as are lots of different spindle speeds.  It helps a lot if you can get the workpiece turning at the speed you want.  My old American Pacemaker (1941) is by no means modern, but it will probably still be usable long after I am gone.