"But I know fuck all about frame design, metals and gear ratios."
Frame design is not that important, what you want to do is keep everything like that simple. But to start off with it's best to use recycled stuff like bike frames or shopping trollys, this way you'll get you eye in with design just by looking for places to mount stuff and get used to the tools and skills needed (drilling metal is not as easy has most would think and welding/cutting metal needs some getting used to, or at least the burns and cuts do). Metals and gear ratios dont take much learning as all you have to do is talk to the people your buying from about what you're doing and the'll go out of their way to help (they've seen it all before so they will point you in the right direction, and even do stuff like cut metal into the shapes you want or track down parts).

If you're buying one just make sure all the parts are there. The frame should still be in one piece and ready to use, put your motor and controls on it and go.

Just make sure you fix the brakes first.