radio/servos - if you get the G4 RTR, it will come with futaba servos that work just fine. yes, if you decide that you want to get serious about racing and need to shave off that last .01 of a second to qualify, then you will want to change your servos out for something a little faster.
however, getting to that point - the point where you're so consistant that your steering servo is holding you back - is a long road and will not happen overnight.
The DX2.0 is what i have, which is basically the same thing as the DX2, just slight redesign on the shell, I think its the most comfortable one i've ever held, but it's up to you and what you think is cozy. It will come with 2 servos, assuming you buy it brand new in the box. I still have my servos in the box, as the futaba's have been fine for me.
changing out the receiver is not that big of a deal, comparitively speaking. It will require that you take the old receiver out, remove the servo and battery wires (little plugs) insert the new receiver, put the wires back in, run the antennae wire and close it all up. all together, on my G4 it took about 30 minutes...
and i was drunk.
again, the title of this thread is 'best nitro RTR for competitive racing' and I have to reiterate; the Team Magic G4 RTR is leaps and bounds ahead of everything else on the market.
you have to remember, that RTR's arent designed to be as competitive as the kits are and you're not going to win the nationals with one.
what you want is something that will work for you - something that you can learn to tune your engine, suspension, etc... something that is durable enough to allow a few slip ups (read: walls for lunch) and something that you can learn to drive fast on a track with.
understand that you will eventually be purchasing upgraded parts, or a new car as a whole, once you feel comfortable in your RTR and need new parts to go faster. You should pick a car that will allow you to add parts to it in the future, as well as be a good 'trainer car' while its stock.
when you consider all these categories, there is really only one choice out there, and it starts with a 'G' and ends with a '4'
I can guarantee, if you get the G4, that you will not be dissapointed.
The G4S RTR has a great engine, a great pipe and a platform that begs to be hopped up... with the eventual purchase of a G4S upgrade kit, (about $189) it will be as competitive as almost anything else on the track.
hope this helps!