300E - Cost effective repairs at higher miles.

[Since66]

This seems to be a good place to get opinions. I have an '86 300E with 152K miles, owned since '89. Only really major repair has been an alternater some time ago. Love it. Runs like a champ, getting approx 19 mpg in city, but now uses oil at the rate of approx. qt per 1000 miles. I know this is not excessive but I also know that it won't get any better . My last compression check showed nothing to be concerned about. My questions: 1) How rapidly will the oil consumption accelerate until it reaches an unacceptable level, say 500 mi per qt. 2) Should I wait until that point to consider doing the valves, etc. 3) How cost effective would spending a couple thou on the valve job be on a $4500 car that is approaching 200K? The car has never seen a body shop. The paint is vg and leather interior is as new. To spend or not to spend. That is.......