Bought an '84 318i automatic from the original owner, a little old lady in Brentwood (down the street from O.J.'s house) with 75,000 original miles eight years ago.
This was for my daughter's first car and she still drives it today with 200,000 miles. Paid $2400 for it and have spent less than $2000 over eight years keeping it running well and looking like new.
I have done lots of work around the car with the Robert Bentley E30 manual, and have never heard of gas pedal access to computer codes. If you join BMWCCA, the local chapter usually has access to the specialized equipment you need to read codes.
A good air conditioning shop will charge about $800 to repair your a/c (probably needs a new compressor and drier at least) and convert it from Freon to R134a. Our air wan't working when we bought the car and we've never fixed it as we live at the beach.
I think I can help you with your high/erratic idle. There is an idle control valve at the highest point of the engine. It has two rubber hoses and one electric connection going into it. These valves get gummed up, and when they stick, so does your idle.
Disconnect the electric plug and loosen the hose clamps so you can remove the valve. Holding it near your ear, shake it up and down - you should hear a rattle like a can of spray paint. If it's quiet, you have a stuck valve.
Get an aerosol can of Gumout or other good carburetor cleaner. Cover all but one orifice with your thumb and fingers, then spray lots of cleaner into the open hole. Cover this last hole with your other hand and SHAKE it violently. In a few seconds, it should free up and start to rattle. Keep adding cleaner and shaking it 'til it's as loose as you can get it.
Shake or blow the excess solvent out and reinstall. When you start the car, idle should go up to about 1500 rpm, then fall back to about 1000 rpm in Park, if you have an automatic.
I did this trick once a year for four years. Finally broke down and bought a new valve - $125. Try cleaning yours first.
Don't get discouraged. These are simple, well-built, almost indestructible cars that will give 250,000, 350,000 or more miles with the kind of care you seeem to be prepared to lavish on yours.
I change oil (Castrol Hi-Mileage 20W-50 and Purolator filter) every 90 days. Have the transmission serviced with Red Line D4 ATF every two years (25,000 miles). Smelled gas one day, replaced cracked fuel line and fuel filters. Recommend PBR Deluxe disc brake pads. Change your Bosch coil, distributor cap, rotor, wires and plugs if you haven't already. Etc., etc.