RE: Replacing ESC and receiver, how do I choose?
well, to answer your basic question, the answer is: it depends.
it depends on what kind of transmitter you have as to what receiver you need to get. They have to be compatible. That usually means you have to use the same receiver brand as your transmitter. (There are limited exceptions) I've been very happy with the Fly Sky GT2 trans/rec I got from HobbyPartz.com. For $20 you get a 2.4ghz trans/rec that works great, no crystals, no interference. For $40 you can get the GT3 with a 10-model memory and lot's of other features like end-point adjustments and anti-lock brakes.
As for the ESC, it depends on what kind of motor you have, whether you still want to use it, is it sensored or not. I have no experience mixing ESCs and motors of different makes. You would have to know how much power your motor draws and buy an ESC with the appropriate amperage rating. Again, HobbyPartz has EZrun and Xerun ESC/Motor combos for very reasonable prices. Your XXXT I think is similar to my Evader in weight and layout. I am running EZrun 35A systems in both my evaders with 4300kv motors on 2S Lipo. Plenty fast for me. and each system cost me less than $100. What YOU choose depends on where and how you drive. Street-running speed runs or off-road bashing?