I have the same engine as you do in my car... 201.028, although mine is an 88.
I thought the car was the greatest thing the day got it and the first thing I did was replace the timing chain, tensioner, and valve cover gasket, along with its first oil change. That made a huge difference in mine and made me love the car even more.
The next thing I did was replace the 4 shocks and I chose the Bilstein Heavy Duty, which are great, very strong, but you lose comfort, well at least here you do, because of the harsh roads. My next move was supposed to be Eibach springs, but because of all the other issues I encountered, it got put off....and that was 2 years ago now.
I replaced the valve seals, because of the horrible oil consumption, and that was one of the best things I think I ever did to this car, well worth it. I replaced the center resonator with an oem part from mercedes and replaced the 02 sensor about a year ago. I bought a sport stainless steel muffler last week and cut out the catalytic converter and replaced it with a straight pipe. It seems to be very quiet, up until last night when I started to hear a rattle, which I have no idea what it is, so I'll have to check that out next.
The head is making noise, I don't know if it is valves, lifters or what, but I can tell you that it is annoying, especially because my differential whines all the time now too.
Anyway, I've changed more parts than you can imagine on this car. I've spent much more in repairs than what I originally purchased the car for, so I can tell you one thing, be careful and calculate before you venture. I'm not trying to sound like a big complainer, but with the money I invested into my car, I could have been driving a nice W126 which has been proven to be more reliable. I wish I knew about this great site then, as I would have done some things differently.
Good luck and feel free to ask me any other questions you may have.