I agree with the other comments. The unit appears to be
in good condition. Even if the membranes and membrane housings have to be replaced,and the pump rebuilt, you are still way ahead of the cost of a new water
maker. (The sensor probe that measures product water
salinity may also have to be replaced.)
The unit can be tested "as is" using the existing membranes. Even if
they do not produce high quality water, a test run will demonstrate correct operation of the high pressure pump and controls.
Prepare a 30 gallon plastic test tank (barrel) of artificial sea water using sea salt
mix available at aquarium stores. Plumb the intake of the RO skid to the bottom of the test tank, and dump both the brine and product water back into the top of the test tank.
Install new paper filters in the raw water
filter housings, open the high pressure control valve all the way, and fire up the unit according to
the directions in the manual.
Wear eye protection in case a hose or fitting on the high pressure side gives way during testing. (Before installing the water maker permanently, have the over-pressure protection devices tested by a professional.)
The high pressure pump should be able to reach 850 - 900 psi without difficulty if it is in good shape.
New generation sea water RO membranes are somewhat more efficient vs. previous versions. They have a higher "recovery" of fresh water from salt water
when run at the same pressure. You may end up with a water maker with more production capacity than the original spec.
in the feed water is the main enemy of RO membranes. Install an oil
separator ahead of the RO unit, and never run the water maker in an oil polluted harbor.