well.... Larry has forgotten more than I know but my approach is to use preventive maintenance in this case.

I replaced my vacuum pump recently without much difficulty. Also am replacing timing chain/tensioner and did water pump last Spring. The only aggravation I had with the vacuum pump was getting at the allen head fasteners that are used to attach it and having a strong enough allen tool to loosen them.

I don't think there are other potentially destructive components to worry about. I know the oil pump could fail but apparently that is extremely rare. The oil cooler lines can blow but are supposed to weep first as a warning.

Different folks take different approaches to this type stuff but at 150k intervals, this type spending seems ok.

I'll let the A/C compressor, power steering pump, and generator go out on their own.

Nic
'85 300CD @ 151k miles