If the comp is shot, get a new one for r12 and drain the oil out of it and add the appropriate amount of ester oil. You will need that compresor with the fittings to carge the system, but it will come with mineral oil for r12 in it.
Might want to check out www.nostalgicairparts.com, he has ne ones for $225 and if you ask, he'll drain the oil and put the ester in for you so it's ready to install ( I buy all my compressors and odd hoses from him, great customer service)

Chances are though, it's fine, first with engine off grab the compressor clutch and see if you can turn the compressor by hand, if so it's not seized,
Remove the electric plug from the switch on the accumulator and run a jump wire across the pins in the plug ( wire harness side, not the switch on the accumulator )and fire it up. Compresor should engage, if you get any cool out the ducts, it probably just needs a recharge. Check for static pressure in the system, hook up a gage set, or at least one of those cheapie quick check gages on the low side with engine off, and then start it up with the compresor jumped and see what kind of presure reading you get
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-------Robert, '93 940t, '90 240 wagon, '84 240 diesel (she's sick) , '80 245 diesel, '86 740 GLE turbo diesel, '92 Ford F350 diesel dually