Building a lathe could be super fun. As someone who has built a few machines, I can tell you it will give you a unique perspective on how that machine operates.

A metalworking lathe, though. I like the cut of your jib. That will be fun, and potentially extremely dangerous. Don't kill yourself, please. There is nothing more terrifying than a lot of mass with a fuck ton of momentum.

Sounds like quite the undertaking. Good luck!