Really only need a clean/dry compressed air supply (so a normal compressor and a filter/dryer,) regulator, accumulator and a piston (connected in that order) for a general purpose pneumatic counterbalance. Just that will be more linear and accurate than a gas spring or coil spring. Most air regulators will bleed off if the pressure is more than a few psi above the set point, so if you don't have enough accumulator capacity, you will bleed air on long downward moves and consume air on long upward moves.