In my opinion you should get the Squishy Paddles and Vortex mod first. After you have them all that you really need would be a nice barrel considering you have a consistent air source, so I'd say Paddles+Vortex, then barrel. I personally have the J&J Edge Kit which I hear is amazing but I am going to test out this weekend.

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A5 - W.A.S E-grip - Polished Internals - Double Trigger - Adjustable Car Stock - NcStar 4 Reticle Red Dot Sight - J&J Edge Elite Kit - Wargear Flexi Paddles - Vortex Mod - X7 Hopper - Remote Coil - CO2