This definitely a question for those who know. Which parts should I order to replace later?
For what they cost I would recommend having a complete first and second stage piston assy with rings in stock. In order to change just the second stage piston, you have to remove the wrist pin to access the bolt that holds it in place. Kind of a pain but doable if needed. Snap rings hold the wrist pin in place. The con rod and 1st stage rings are a wear point along with 2nd stage seals, Easier to just drop an entire piston assy in place. Just be sure when inserting the first stage rings into the jug that you have spaced the gaps in rings around the diameter so as to prevent blow-by.
I would also get the gasket set that goes between the crank base and jug and the second stage cylinder.
My pressure gauge blew the rubber plug out of the side the first year, replaced it with a better gauge, and used a metric to NPT bushing so I could use a good glycerin filled gauge with an 1/8" NPT thread. My new gauge is larger and easier to read and goes up to 6000 psi.