You should be looking at the 'dealbreaker' items first.
A/C comes to mind.
Nothing in the suspension/steering is that bad taken alone, but getting stuck with a full front or rear redo can indeed add up to big money.
Find out what has been done already, if you can. If it has been maintained, you should not get stuck with any big ticket repairs.
Have it checked out. Could have a lot of good life left in it.
Realistically, you are looking 12 year old car, and should adjust your expectations accordingly. I think you would find that a 12 year old 300E is more solid and reliable than most other 12YO cars. Also more enjoyable to drive.
Using your example, would you consider a 12 year old Grand Prix? Dead reliable? Fun to drive. How's that depreciation coming along? Consider total cost of ownership over time. Lifecycle costs, as they say...
May cost more for the first few years to proactively maintain the old Benz to keep it reliable, but what will the newer GM product really cost you in the long run?
1986 300E 5-Speed 240k mi.