Id start by searching the various ads online and see what kind of prices what you're looking for demand. Once you've got a idea what the vehicle will cost, then you can proceed based on that.
One tip I would give is to not buy something that's got a lot of rust repair, or is not close to complete. Cars with missing parts will kill the cost of a build trying to replace things. As far as the drivetrain is concerned everything can be missing, as I doubt you'll use hardly any of it.