Tower's good. I got a lot of stuff from them back in the day. Go to the local book store and geta copy of R/C car Action - they will have adds from all the big shops.
You're gonna be plunking down several hundred bucks for something decent. Don't for get all the add on bits - most of the better car kits are only the car. No motor/engine radio gear etc.
If he was zipping around the back yard, im assuming he is going to lean towards getting a truck then a kysho. look at about 500 for a truck and another 1-200 to get you started. You will need a lot of glowplugs, glowplug igniter if your car does not come with one, , new tools, batteries, charger, fuel, fuel bottle, and maybe a good bag to haul everythiing in. YOU WILL NEED NEW PARTS as i will not be surprised if you brake an A arm or something by smashing it into a curb at 40mph.
Buy the things to get you going at tower hobbies, get the spare parts from your local hobby store. When you have a feel for what you will need, then internet shopping for all your parts will be more comfortable and familiar.
