Are you able to take a few evaluations on compressor prior to purchase?

These are not scientific and maybe not the best of testing but should tell you if it is broke or should run.

IF so
HYDRO test
measure circumferance of tank. Tape measure will do - especially if you can get down to say 1/32 increments.
Install vent to let air out of tank. Attach quick shutoff on garden hose. Hook garden hose to tank and fill tank COMPLETELY full of water - ideally no air at all. Would be handy to have gage on it use say 50 to 75 psi gage.
Shut off hose then shut vent. Crack the valve and pressurize tank.
Record pressure. Look for leaks - especially your test fittings. Try to maintain the same pressure. Remeasure circumferance. If no change that is a good tank for sure. I would expect some change in circumferance and would not worry about say a 1/8 change - all tanks will change. What you are really looking for is bulging.

Generally they fail like a hot water tank small hole that gets bigger and bigger. Rusting by itself will generally NOT cause a tank to blow up. They can fail and can explode but that is for other reasons than just rusting - ie very rapid pressurization, dieseling etc BUT that is generally with high pressure air ie 3000 and higher.

Compressor - if you can put known length of pipe capped and whose volume you know on discharge - install a pressure gage - probably 0 to 20 psi. Turn compressor a turn and record pressure. If no big result then try 3 to 5 turns. If you are confident of your readings then you can use the known volume of your test pipe and calculate volume per time (CFM absolute) and compression ratio
Vol / one (or 2 or 3) revs and
PVT = PVT

If it generates pressure and can hold it you have a pretty good idea rings are OK and if it holds pressure valves are OK.
I would not be surprized if it does not hold pressure for more than 10 second.

Motor go across line leads. This size is more than likely capacitive start and cap run. I would guess 5 ohms or so is OK - no ohms something is wrong probably open winding. Check from one line lead to ground on motor case should get better than one mehohm on a good motor. If these are OK then you are probably in good shape. If you have doubts take to motor shop and have em test run it. Cost maybe 50 to test but that motor to replace is probably 2 to 300.

Compressor shaft look for endplay and slop in bearing. IF they will let you drain the oil and look for metal in first part of draining - coffee filters are great for this.

Dan Bentler