Just completed that same function on my 81 300TD-T (123 body).
Only tough part was getting the old compressor off. My car has the GM R4 mounted low on the drivers side. If you have the same config, it will be much easier to get at two of the three attaching bolts if you have a ratchet with a swivel head. I think the bolts are 18mm. Be careful of where the spacers go (if you have them) as they are not the same thickness. On mine, only two bolts have spacers. The receiver/drier and expansion valve are very easy to get to and replace. My cost was $225 new compressor, $42.00 receiver/drier and $18.00 expansion valve.
Dan Taylor/ Tulsa, OK MBCA '71 250C/'81 300TD-T