These cars were one of the best of the MBs ever made...but what you are experiencing is a vehicle that is getting pretty long on the tooth, and needs some TLC to get it back into tip-top condition.
Like Benz300 said, you should consider budgeting repairs in smaller increments...parts are pretty robust, and many are designed to last a decade. So it you do the suspension stuff for instance, you shouldn't have to worry about looking at it again for another 150K...
..with the exception of the techs here who can keep their rides at 100% all of the time, you are not alone. There are other commitments that prevent me from attacking each and every anomaly that plagues my vehicles...so I have to prioritize and do the critical stuff immediately, and then figure out which of the other repairs I will do later. I have many that have required attention for years (like a sagging headliner), but I just don't have the time.
The other issue is, what other vehicle would you consider as a replacement. You might consider a used Accord or Camry as an alternative, as both are considered fairly reliable.
Your wallet will probably breathe a sigh of relief, but understand that very few vehicles are built with enough robustness to last several years of regular use...so expect broken plastic pieces, torn seats, faded paint, and surprisingly, when something DOES break, parts and labor can exceed what it would have been for your current vehicle.
If you decide to keep your car, try not to look at all the potential problems in their entirety...that will overwhelm you.
Also, pick your battles and do some research...the head leak for instance. Some head leaks are not directly related to a failing gasket...have you seen oil in the coolant or consistently hot engine temps? And is oil leaking from the head or valve cover? Front or rear head area?
I'm saying this because there are other causes to oil leaks that aren't necessarily a symptom of the head gasket...and far cheaper to solve.
Anyway, that's just one of the ways you can go about determining what to do at this point.
Good luck on whatever decision you make.
