I think the most important question before starting any build is deciding what motor to use. That single decision will drive critical decisions on chassis, wheels, brakes, etc. It will also determine how much $$$ you'll dump.
A China two stroke is great starting point for most folks, they'll bolt into almost any frame and with the modest power the bike does not need to be particularly rugged. Because these will bolt in, most guys can do their own build. There are tanks available for Worksman and Schwinn straight bar frames that will bolt right in and make you look like a pro.
Morini's are better, but they've gotten hard to find and remain expensive. Plus these don't bolt in and without fabrication skills, getting them properly mounted can be a real challenge, plus they need a stronger/roomier frame. Likewise with the smaller 4 strokes.
And then at the far end of the spectrum are the big 200cc four strokes. These are best left for custom chassis. Deep pockets required.
Good luck!
