There are lots of things to consider on these cars....they all have the potential to run a very long time. It is a question of maintenance and how much you want to "invest" in the car.
1. Timing chain/tensioner/rails - should have been replaced at 95,000 - 100,000 miles. The chains don't usually break - it is the tensioner that fails and then the engine is scrap. This will cost about $2,000 to get fixed - maybe less, depending on mechanic. $5,000 if you wait until chain breaks. Is engine compartment clean?
2. Climate Control. AC in these cars weren't strong to start with and didn't improve with age. Make sure the AC works (really works - not just a recent charge). Check for conversion from R-12. Also check for air direction (does air flow change from floor to defrost).
3. Oil leaks - these cars were meant to be driven and don't react well to sitting - the seals just shrink from lack of lubrication and things start leaking. The main seal between engine and transmission is a big one, as well as the seal around the oil level sensor and oil pan. Main seal and oil level sensor are somewhat expensive - $1,000 or so.
4. Transmissions have not been a problem on these cars, but they MUST be serviced. Check for any leaks, hard shifting. If the gear selector is loose, not an indication of trans problem as the bushings go out....$100 for new bushings (installed)
5. Overall condition of the car. Do numbers on body panels match (meaning is the VIN on all panels - it is a sticker that is on all panels. Replacment panels don't have the sticker - so good indication of replacment panels - not a big deal if work was done right, but can indicate accidents. How is interior? If leather is worn, not a big deal - an auto interior place can re-dye or replace panels in the leather for about $500 - $1,000 for the whole car (the perforated leather is more expensive). Wood is easily replaced and not terribly expensive (don't even try to refinish yourself - you will never get the Mercedes finish off the wood).
You WILL use more gas than you do now - bigger engine and bigger car.
ENJOY!