Hey funky,
The C-rating on the pack is only important, I think, in how warm it gets in an individual system. There are ways to calculate it, phmaximus is good with that, but I feel a good 30-40C pack is all most of us need as "sportsmen". Some motor systems may require beefier packs with higher C-ratings. You'll be fine with most of whatever lipos you get. As for a specific recommendation on HPs site in your price range, I'd look at this:
http://www.hobbypartz.com/98p-30c-40...se-direct.html, but for a longer runtime and not that much more $$
http://www.hobbypartz.com/98p-40c-50...se-direct.html Having said what I did above, anecdotally I found using these batteries in that motor system, the 40- & 50C packs stay a little cooler, and therefore I think should last longer.
Looking for batteries on hobbypartz site can be frustrating sometimes. Their homepage Directory doesn't always take you to where you think. They don't update their links very often apparently.
What kind of battery connectors are you using?
I also
rescind my endorsement of the EXI D227 servo. The second one has now crapped out. I may take the time to get a replacement, just one more chance, but the trusty Hitec is proving serviceable and reliable.