I would get a tank based on (A)how much paint you shoot, (B)how much of a gas hog your gun is, and (C)availability of 4500psi fills. Most if not all of the 48's should be lighter than a full 16oz CO2 bottle, and I think you could handle a 68 as well. You won't be 13 forever anyways. I don't know much about the drop you mentioned, or the exact HP system you are looking at, so I won't comment on the drop. Have fun shopping!
Ilove this game!