Stick with the stock engine untill you are happy with the handeling of the car. Adding more power will make any handeling problems worse.
The Fix, threaded shocks & side brace should be all you need. Your track should have a class to suit .15 engines & RTRs so practice your driving & set up while saving your cash, then when you are ready for more power upgrade to a hi performance .12. Have a read of the NTC3 thread & you will find plenty of info in there.
BTW the RD pipe does not work well with all engines. I like it with a Sirio but the Werks pipe is much better with a Rody. The AE duel chamber works fine for most applications as well. Dynamite is best left on the hobby shop shelf.
It is a great car when set up properly, get the XXX main or Hudy set up guide & experiment with adjustments, this will make you much faster than any bolt on upgrade. Have fun