Pick the right first machine, you don't want a non-working machine where most of the replacement parts are unobtanium. And it's not just cleaning and tweaking, in many cases you have to replace parts, plastics and rubbers.

And start with getting a machine work in playable condition, restoration should come later

Oh, and first rule of fixing. Take pictures, lots of pictures. Putting pins back together can be hard without good documentation (and there are thousands of parts)