HPC is the obvious choice, but they are very expensive especially for racks, I should know.

You need to work out how much travel you want your axe to have, what cylinder you are using, find a suitable rack to fit the cylinder. Then work out what size pinion you need to get the travel based on the MOD or TDI of the rack.

Take the stroke of the ram and work out what size pinion you need based on the stroke as a percentage of the circumference of the pinion. So if the stoke of 100mm and you want 120 deg travel (say mod1 for ease of calcs) you will need a 100* 3 as the circumferece (120 being a third of 360) so 300 /(2pi) = 47.7 for the rad of the pinion and 96 for the dia and teeth for mod 1. But it probably wont be mod 1 anyway.
But Im sure you knew all than nonsense anyway, plus i hope I got it right.

As far as pneumatics, bottle, reg, buffer, valve 5/2 or 5/3 valve(same as thz) then ram, plus fittings and pipe. All rated to the pressure you are running at, not forgetting a LP relief valve.