Just a note on the ion, i used to have one running off c02, with an anti siphon, its fine ..... until you really start upping the rate of fire, then it just looks like a big icicle, damn thing ended up sopping wet.......
if you want to use co2 and dont have an air supply at your local site i would suggest a mechanical gun so look at tippman 98's these can be upgraded easily but are murder on your finger if your trying to shoot it fast or try looking at the autococker trilogy range i have a trilogy pro and it is one of the most accurate mechs i have shot but if you have air available then the other posts have things well covered