Buy a well sorted out race car with a log book unless you really want to go through the process of building your own car for whatever reason. You will save a huge chunk of money and time. I'll buy something cheap and develop it slowly mentality doesn't change that fact. Make sure you don't end up with someone else's failed project though; have someone who knows what he is doing test it on the track, etc.
Check out some NASA GTS cars here: http://www.racingjunk.com/category/4...cing-Cars.html