You need to search on this forum for ready to run ZX-5 tips, a lot of the parts from the original kit were changed to plastic to keep costs down, but just for stupid fast, you will need to change the motor, as the numbers get lower the motor gets more grunt, we used a 10turn brushed then went to a 5.5 brushless, only slightly faster (5.5 = approx 9turn brushed). To fit a faster motor you will have to change the speed control (esc) to something that suits the motor, I believe the RTR is limited to 15turn, some of the brushless esc's are a tight fit under the body shell.
Once you start to go stupid fast you will break parts, more fun is to find a local club and go just fast enough round the track to enjoy yourself (you will still break parts), you will also find a friendly, helpful bunch of racers who will help and give you lots of advice, motors, esc's, tyres, shocks, batteries...............
The Kyosho ZX5 is a very good place to start, will run well and will be competive. have fun
Jimmy's review is well worth reading a few times,
http://www.oople.com/rc/index-reviews.html