All of these items depend on how much DIY you are able/willing to do. Don't ignore the fact that this is a 20+ year old car and will need work.
1) AC- sounds like a suspect failed compressor. ($300 + about $150 other parts) Flush of system and recharge. If not done DIY, can easily reach $1000.
2) Rust - The mysterious openings you mention are jack points, which is part of the structure of the car. Beware! You will need to find out exactly how extensive the rust is in this area.
3)Power windows - elec. and may just be fuses.
4) Door locks - vacuum leak is probably the culprit and that portion of the system may be plugged in order to provide for fuel shut-off.
I would definitely suggest a pre-purchase inspection from an independent who is familiar with MB's of this vintage.
