Read this thread, if you haven't already:
HPI Sprint 2 Mods - Making a silk purse from a sow's ear.
Marv has put together a great resource thread for most things HPI Sprint 2.
For a radio, you can get just about anything 2.4GHz and it will be fine for bashing. I use a Futaba 4PL and I race. It might be overkill for your needs.
For a servo, I have the Spektrum S6070 low profile servo in my cars. It fits nicely in the Sprint 2 chassis and is reasonably strong & fast and reasonably priced. http://www.spektrumrc.com/Products/D...odID=SPMSS6070
I would also invest in a glitch buster cap to prevent brown-outs and glitching due to voltage sag during acceleration and turns at higher speeds.
Replace your ballcups with these: http://www.losipartshouse.com/servle...-molded/Detail
I found the RPMs to be too tight and caused binding. The were too long also and required cutting to fit.
I hope this all helps!