It better be cheap because it's not worth what a shop will charge to correct the known problems, let alone the unknown.

I believe replacing the heater core is a take-the-dash-apart affair as it is with 124 and later series. Not particularly difficult work but plan on a solid 20-30 hours with all the labeling and trial-and-error identification of what you forgot to label. If prices are similar to the 124 series, you're looking at something like $500 for a heater core and vacuum pods (which you should replace while the dash is apart). You might luck out and find a bad monovalve will get heat working again. Then it's more like $50 and a half hour.

Power window switches are a scrap yard part or check the parts forum. Sounds more like a relay problem, though.

I figure the center bearing will take a first-timer 8-10 hours and possibly a trip to a machine shop if it needs a press and you don't have one. You might replace the flex disks while they're accessible.

Idle stability could just be a bad overvoltage protection relay or engine rpm sensor (does the tach work?). The should idle or can be made to idle mechanically if it's a problem with EDS or ELR.

Are service records available? History of suspension work? Rust?

edit - there's always a used heater core. Don't get used vacuum pods!

Sixto
87 300D