Don't want to rain on your parade
Folks, I work for Dow Chemical and we bought FILMTEC back in the 80's. I sold the first units to equipment manufacturers in the S.E.
The major cost of the unit is the membrane. The TFC membranes used in most models cannot come in contact with chlorine. Also if a unit has sat idle for some time the membrane itself will dry out and possibly not work as effectively as it should. Use can always test with a TDS meter or conductivity meter though.
Therefore when purchasing a used model, you may always want to consider to at least replace the membrane or be sure you know the circumstance of the unit (still in use, always Carbon protected (absorbs the Cl2))
I'm not trying to sell more RO units, I'm far from that business today. And when I restarted my tank just recently - I did not use my old RO membrane or Carbon.
Just a thought - Dale