Method for Starting a Project

There really can't be an answer to this question without knowing a lot more, but I'll build a little on what a few others have already said. The way I see it, it REALLY depends on 3 things. 1) How far are you taking it, 2) What are you starting with, and 3) How much can you afford to spend.
If I bought a decent driver and don't plan on a complete restoration, then I would probably consider engine, drive train and suspension first. Then I'd do other things as I could.
If I bought a car (in any condition) and planned on a complete rebuild/restoration, I would do bodywork and rust repair first, then paint, followed by suspension so as to make a roller. Then driveline and interior would be last.