To go 40mph in a truggy, 4s is plenty - and will give you gearing options. So I don't think you need to use 6s for what you want (although you certainly could).
Will the CC Jammin X1CR buggy conversion kit fit the truggy? Or at least the battery trays? It's still available on clearance:
http://www.castlecreations.com/produ...sion_kits.html
For batteries, you'll get plenty of opinions. If you go with two 2s packs, these would probably my first choice (as a budget-conscious basher):
http://www.hobbypartz.com/98p-40c-50...se-direct.html
And you could get 2 sets of these on your budget.