Like any used car, there are bound to be problems associated with wear and tear consistant with usage.
The W126 has one of the most excellent built record in that there arent many electrical components thus making maintanence a better task than the later versions of the S-class.
As BILL put it in his checklist the things to look out for, I have this to add:
1. check for rusting beneath the floor mats and in particular the bottom edge of all doors and boot area.
2. oil leaks - another very common problem;
3. smoothness in transmission change and check for leaks around the gearbox;
4. steering freeplay - for this age might be due for a change of steering box;
5. engine mountings and general condition of the engine - does it start well, and idling fine?
6. alignment of doors - as these cars are very sensitive to windnoise if one door is out of alignment;
7. condition of tyres - uneven wear would indicate either tie-rods or undercarriage suspension problems.
8. fuel pum and relay - does it hiss loudly when started?
There would be many more to consider.
But I would encourage you to get one of these marques as it would be fairly difficult to get these cars at such top end value.
Another bonus - its durability and proven reliability at very low cost. You have another advantage when it comes to parts: new and used.
Do check out this site for further info: www.w126.com