Frankly I would start with one of the small Sieg lathes, also sold under other names, rather than try to make your own if you don't already have a lathe. But not too small. You get a reasonably accurate bed, spindle and slides as well as the motor drive. Backlash is not nearly so critical in a lathe so you can live with the normal feed screws though ballscrews are nice to have. It is easy to underestimate the rigidity you need for a lathe as well as the fit of the slides if you want to turn anything metal. Lathes are the essential machine tool.