I'd have to recommend a conversion to R134a refrigerant for this car, along with fixing whatever leak there is.
The factory kit for the 740s is only about $70, and it's a straightforward installation. A Variable Orifice Valve is supposed to help efficiency- see the 700/900 FAQ for lots more on A/C.

You're likely to need 1 or more major components- a compressor, condensor, or a hose (all of which run $100-300 for the part). After it's mechanically sound, an A/C shop can pull a vacuum to evacuate the system then recharge. Hopefully all will go well, but I'd realistically expect it to take two tries- that's just the way old A/C systems always are. The shop probably sticks a big disclaimer on every job, and there's a good precedent for it.

Realistic total price for the work is $600-800. If it gets done for less, consider yourself lucky.
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Rob Bareiss, New London CT ::: '87 244DL/M47- 228K, 88 744GLE- 212K, 91 244 183K-SOLD! Also responsible for the care and feeding of: 88 745GLE, 231K, 88 244GL, 146K, 87 244DL, 239K, 88 245DL, 246K