Just my $0.02; I have a similar project in mind - and similar experience
I bought a "Graupner Punisher Rock crawler" for this purpose;
1 - It was on sale
2 - It has electronic motor control; the whole car is controlled by servo signals only - no mechanical throttle parts.
3 - When I take off the (silly) "chassi-cover", it has a good aluminium frame where I can attach a platform for all of the electronics. I thought I'll go for a large plywood or thick plastic sheet that I attach to the chassi-pins.
4 - Since I'll make a detachable platform for all the electronics, I can easily convert it back to a car and play with it
I don't need to do anything to the base car (just unplug the receiver's servo cables and plug in my own).
There are tons of servo controllers out there, I just need to figure out which ones I can get hold of. Then I thought I'd add some Sharp IR distance sensors, an accelerometer, an multi input ADC etc. etc. I probably need to add another battery too...
I just got my Pi yesterday
, so the project has gained a little more momentum now!
P.S I can recommend the book "Robot Builder's Bonanza" - got tons of useful info.