The pre-built kits are known as RTR (Ready-to-Run). They come with everything, including radio controller. Just buy batteries and a charger, and you are typically set.
It is more fun than you can imagine. I reduce the settings and let my 3 year old drive a 1/10 scale car.
Many electrics are available as RTR, but I think there are also some fuel RTR's also.
The company HPI might make a separate Ford GT body shell, that is likely compatible with several companies product.
I don't personally know anything about the RTR offered below or the manufacturer, but it gives an idea of what is available:
http://www.cenracing.com/cars/ct5/ct5.html
I do own an Associated Racing TC3 1/10 scale, which my lovely wife surprised me with as a gift. I've never owned a quality R/C, all my previous were the $30 and under variety. My wife asked the local hobby shop guys what to get and they recommended electric (vs. fuel) as a beginner. I like it. I painted the body shell estoril blue, to match my M3 Sedan I had at the time. It's quite durable too, the shells are much easier to paint then you would think (you only paint the inside of the shell, so the paint never chips and the finish is perfect, if only it were so easy with 1/1 cars) and strong and flexible. I'd love to build a nitro car sometime...I hear it is a great learning experience.
As far as gas and electrics - that's up to you. Talk to a bunch of people and see what you like. I've raced indoor electric and gas karts, and I hate the electric karts. But for a first R/C, I am plenty pleased with the electric (no maintenance, no noise, take it anywhere). They are reasonably fast too. Of course, it will be nice to get a gas R/C too, when I take the time to build it. Or maybe I will try to follow Shadowman's lead and save it as a father/son project in a few years...