I can't give you a complete answer, but I can tell you my experience. My 300TE wagon has 155k and the head gasket just began to leak. So, I'm doing the head gasket, timing chain (sprockets were essentially perfect, as they often are), water pump, lifters (a couple of them were bleeding down), valve guides, seals, and a valve job.
My brother-in-law went thru the same thing with his '88 wagon at 225k mi (approx); his got more miles/yr than mine, but time-wise they went at about the same age.
If it were my car, I would go to 200k mi. before I looked to pro-actively start ripping that motor down and replacing stuff.
My total cost I expect to be around $1,500, including all parts & labor (remember, the water pump alone is several hundred $$)
Just my $.02...good luck
All the best, Michael