I am thinking about putting some Preventative Maintenance into my car when I get my money back from the government. I am willing to spend about a $1,000 to get things done. I need advice on what you think I should be doing. Some background first.

The car is a 1983 300D with 225,000 miles. Both the Glow plugs and battery was just recently replaced along with the oil. Tires seem fine in good shape.
I had a tune-up done at 198,342 miles, all fluids were changed a few other things were done. They didn't want to touch the transmission fluid though.
Brakes were done at 187,760 miles


Possible concerns I have:
I know the transmission probably only has another 50,000 miles left in it. And I know this will cost me more than a $1,000. So I want to try and hold out for a while.
Valve adjustment was not done with tune-up.
My AC doesn't work but I have another car that has it..


I do want the engine cleaned again.
Maybe have the timing chain looked at.

My thought was to take it to Mercedes which I normally do and have them take a look and see what they think. I know they are expensive but I have been to one or two other people and have had a few additional problems after I had it serviced.

Any help would be appreciated.