UH... do not assume anything you don't have to...
that could be a disaster....
take pictures of the fittings on the AC system...
and look for tags which might indicate that it was changed to R134a or some junk put in it.. ask the former owners if you are able to also...

Yes, you should look to see if the compressor is engaging... see if the CENTER portion turns after you hear a click..
often two people are better than one.. one inside with the controls...and you looking under the hood..
There are lots of threads about checking and hooking up the compressor for a very short time to infuse the refrigerant.. but that move is only after you know which refrigerant..and whether your car system is able to engage the compressor on its own at this time... the implication being it has enough pressure to override the low pressure safety cutoff switch...

__________________
1980 240d ,

chain elongation, cam marks reference:
http://www.peachparts.com/shopforum/showthread.php?threadid=10414

http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html

evap fin cleaning:

http://www.peachparts.com/shopforum/diesel-discussion/156207-photo-step-step-post-showing-w123-evaporator-removal-1983-240d-1982-300td.html?highlight=evaporator

A/C thread

http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html