Don't buy a tank until you figure out where you are going to get it filled/exchanged. If you will fill at a paint ball store then buy whatever tank you like. If exchanging tanks (Airgas, Nexair, etc), then there is no use buying a tank. Those places take and keep your tank. Just buy a full one from them the first time you go.
I recommend a 5# tank (or bigger) even with small kegs. The price to fill a tank is about the same regardless of size and the places you have to drive to get co2 can be pretty far (not usually nice neighborhoods).
Finally, I bought 2.5G kegs from AIH. brand new for $75. I love them. Morebeer has 2.6G topedo kegs for $80 new.