I bought an '82 300SD several months ago as a veggie oil conversion project. One of its many deficiencies is that the A/C compressor doesn't have a belt running to it -- judging by the overall condition of this vehicle, I'm assuming that the A/C stopped working many, many years ago.

It turns out that the best place to mount the veg oil filter is right where the A/C hose runs up along the driver's side inner fender. Since A/C is not essential for me, I'd just as soon remove that hose if I can. However, I see that the A/C plumbing ties into the radiator -- if I depressurize the A/C system, will that have any effect on the engine cooling system? They should be two separate loops, but just because they should be doesn't mean they are.

I would not disconnect the AC system as this keeps the car dehumidified in winter, not just cool in the summer! Don't assume that because the belt has been off that your system is shot. I've been pleasantly suprised and if your compressor turns, it could just be a matter of replacing some "O" rings and getting a recharge! I'm sure if you get creative that you will properly locate all the necessary space for your system.

I'm sure he's not too worried about the humidity in NY in the winter...

I miraculously made it through a Minnesota winter without A/C and the humidity didn't even bother me!

The A/C system does not connect to the cooling system. There is another unit in front of the Radiator called a condensor that is basically (in very simplistic terms) a radiator for the A/C system. If you have your heart set on taking the A/C system out for the room you will not have any impact on the cooling system at all.