Big bit of pipe a small powder extinguisher bottle can make a good donor for a buffer tank.

No gauge requirements.

A single port valve is all you need to purge the system .......( its normal practice to close the bottle supply valve first )...but a 3 port that shuts off the supply as well as dumps the system is good.

because restricting the flow will restrict the operating speed and power
You may not notice any speed reduction due to poor flow if the ram is in an unloaded state.... to slow the ram down for testing restrict the extend exhaust on the 5 port valve.