The 500SEL was available in Europe in 1980. Didn't make it to the US officially until 1984, and probably only because so many gray market cars were taking sales from the 380SE/L.

Does the VIN begin with WDB126037 or WDBCA37?

If you hear clicking when you turn on the ACC then it's probably not the fuse. However, the first thing to do with any used MB is replace all the fuses regardless of how they look. Should cost you all of $5 from an MB dealership and probably closer to $2 from a VW parts store. If the fuses don't fix it, you probably need a new blower motor (~$125) and a freon charge. Don't get roped into a R134a conversion. R12 is not illegal and the W126 AC system was marginal when new and will likely be borderline pathetic on R134a.

I'm guessing your car has ACC. I've never seen a 500SEL with manual AC controls.

Idles too high? You don't give much to go on but my psychic powers say faulty idle valve or vacuum leak.

Sixto
95 S420
87 300SDL