At that age, it's generally suspension related maintenance that are the big cost items. Struts, springs, bushings tend to start drying out / wearing out if they have not already been replaced.
The guibo (flex disc between the transmission and drive shaft) dry rots and starts to shred.
On the inside, the door panels can start to show their age, sometimes the leather and plastic "skin" starts to separate from the door panel.
Here's a list that has been compiled over the years. These are problems to look for. It does not mean every car has these problems. Some items are normal maintenance, others are more critical/expensive:
Driveline:
- Water pump will need replacement because of the plastic impeller thru '96 (M50)
- Radiator - look for cracks in the plastic neck - replace ~100K miles to avoid failure while driving
- Defective ignition coils in some of the '91 and '92.
- Drive belt tensioner
- Profile gasket on the earlier 318's (M42)
- cracks in plastic thermostat cover (M50)
- sticking thermostats (M50)
- Manual transmission (ZF model) pops out of 1st or 2nd gear under load
- Guibo (transmission flex disk "universal joint") cracks/dry rots ~100K miles
Suspension:
- Outer ball joints go bad
- Rear upper shock mounts go bad
- Shocks/struts need replacing after 50K - 70K miles
- Control arm bushings crack/dry rot ~150K miles causing minor alignment, handling and tire wear issues
Interior:
- Window switches
- Door panels seperate because of insufficient glueing
- Door panel fasteners break
- Dashboard vinyl can seperate over the center vent
- Seat heater elements break
- Glove boxes misaligned - especially earlier models
- chipping trim around the tweeters in the front door panels
- sunroof rattle
Exterior:
- Plastic headlight covers get foggy over time
Misc:
- brake light switch
- squeaky clutch pedal bushing
- Electrical contacts on lights corrode or get dirty and need cleaning/tweaking
