An A5 will run on Air & Co2, if youre firing high rates of fire - with an egrip or correctly set up RT you will want to use Air, to stop the internals freezing, the plastic parts cracking and the oring perishing etc etc, most sites will be kind enough to let you use one of there bottles whether it be air or co2. Your always better off using air as its a cleaner, regulated supply.
Upgrades are absolutely endless for the A5 with most being a waste of time, and alot of aesthetic ones which will weight the marker down and make it a bigger target to hit in the process of making it look "cool".
Generally you will get told upgrade when you find a problem with something, rather than spaffing £100s down the pisser on pointless upgrades that everyone swears by.
Ive spent a fortune modding an A5 in the past and saw very little improvement through any of the "upgrades".
If your cyclone is splitting balls - get some squishy paddles, but also check the hook on the body and make sure there are no ruff or sharp edges.
Dont bother with any front or rear bolts, make no difference.
Dont buy any TechT stuff, its all over priced crap.
Polish the internals - easy to do mod for free.
Response triggers are meant to be used with Co2 and not air, so they are very fiddly to set up right using air.
Egrip, worth getting if your getting out shot
Barrel kit - theres a A5 freak kit for sale on here, best thing you will ever get, you can buy different threaded backs so that you can use it on any other marker you buy as well.