Yea, go with the CO2 unless you can get a scuba tank and the fill for a very good price. I really dont know how much psi a scuba tank can hold but from what i have herd they dont fill them over 3500. if you got a tank such as a 68/4500, it wouldnt fill it all the way. Go with the CO2 since it will be easier.
Also, do you go to any fields?
Sorry for the late reply, but no, I just play recreationally, playing woodsball and playing in large open fields (one of my friend's dad is a farmer, and owns A LOT of land). I'd like to start to start playing in paintball fields but I'd like to wait till I get into it more, plus I have basically no means of transportation.
Azodin Kaos. Better in every way over the two guns mentioned, and lucky for you its only $80. As an added bonus, guess what it comes stock with... an expansion chamber. Or... get an Envy, same price as the A5, and is capable of 11bps full auto, and dosent need an expansion chamber since its regulated.
But if your looking at milsim guns only, then your royally screwed. The cheapest and best woodsball gun is the G1, which as you have said is too expensive.
I suggest you save up more money if milsim is your aim, just do anything to avoid Tippmann.