They're cheap enough - just buy the already made packs in whatever "S" you want. For support equipment, Hobbyking is good enough. One less thing to do, since you really should be soldering both main conductors AND balance leads.
A 3S probably will run the pump, but slowly. Why not just go with 4S? It would be over voltage but not super extreme....or go with the 6v version pump, and the even cheaper 2S.......