Price varies on size and options substantially. I have a mini lathe that I got for 650. The lathe has a 7" swing, 1/2hp, its very small. But good for like you said, little bushings and shit.
The next size up is double the price, but for good reason. 10" swing, 3/4 hp motor, geared spindle vs variable speed controller.
You're going to have to look at what guys generally convert. Which machines have the most potential with a good sturdy frame, a good bed design, work capabilities. The one he has on that site, I dont know that I would try and convert that to an nc. Its a very cheap lathe(I have one similar to that), but I guess if you have the time and you're willing the work out the little quarks before you start the conversion. Then fuck yeah.
Sorry I haven't really told you anything. But that is where i would start, kind of go by what you want out of the machine, then look for one that will accompany it the easiest.