oooh, im interested to see this take off!!!
nitroholic has given you some sound advice already, but ill add my opinion too,
if your set on 1/12th have a look at a tamiya mo4l chassis, long, thin and rwd, you will still have a lot of work but the long "tube" chassis is there and the bits that hang off it.
or 1/10th would be a tamiya tl01 chassis, long, thin but 4wd (you can remove the propshaft and still run rwd) again same principal as the other chassis.
this way wheels and tyres similar to what you want will be out there somewhere, still plenty to keep you busy and parts are peanuts for those cars.
and at the end you will have a car thats usable and unique.
really looking forward to this mate!! gaz.