rtr = ready to run.
Generally, all you do with those is put batteries, glue the tires (depending on the manufacturer) and go. Of course you still have to break in the engine.
Personally, I don't like HPI products. To me, they seem to be a pain to work on. They seem to toy-like, but that's just me.
I like Team Associated. My ntc3 is easy to work on and parts availability is great as is HPI.
If you get a kit, then you'll be paying more than a RTR. Kits are good because you will know the car inside and out, you can choose your radio and engine, but you do have to build it. If you're not really into building or you just want "instant" fun, then you might look into RTR.
Some RTR's have unpainted bodies, so you'll need to paint it and cut the body out. But i think i've been seeing more prepainted, precut bodies with RTR's
One thing you might do, which will help you better find a car, is goto your local hobby shop and see what kind of manufacturer parts availability they have. That will help you narrow down you choice of cars. You don't want to get a car that you have to mail order parts, that can put you out of commision for a week, but hey, if you're patient (i'm not), then mail order might be alright.
Hope this helps