I prefer the Krypton 85 idea.


On solenoid valve. The bigger the valve, the faster it can feed the ram to full glory. With the size of ram you're contemplating, industrial standard is 1/4" bsp in and outlet. That's restricting in itself.
Most FP machines use 1/2" bsp inlets.
You can find 1/4 bsp valves from "known brands" easely enough on Ebay or such, affordable. Otherwise you could try Chinese copies.
I would use single acting. Less complicated, less parts, less gas used per action. So what you can search for is a 1/4" bsp 3/2 valve. Return to be done with bungee cord.

The braided stainless steel hose. Why? 1 paintball bottle, regulator (best bet, a Trevor regulator) directly attached to that. From there a low pressure hose to the buffertank, and that can be a small hose. Buffertank, valve and ram connections need at least a 8mm hose.