All engines consume oil. A quart every 4-5K or more is great. A quart every 2K is so-so, but within normal limits, especially on these Mercedes inline sixes. So you started at a quart every 500 and now it's 1500-2000. If it were me, I would have ordered a valve seal change, which is relatively inexpensive. Once the valve springs are off a good mechanic can tell how much valve guide wear is evident, and at that point you decide whether to just continue with the seal change or pull the head for overhaul.

If the shop told you that a "valve job" would eliminate oil consumption they are either fools or shysters, or maybe you just misinterpreted what they said. All engines consume oil as it is needed to lubricate the valve guides and rings and a little always slips past. It's a matter of what is acceptable, and I would not consider major work (especially on an old car unless I was restoring it) until consumption got down to well under a quart every 500 miles.

A friend of mine bought a new 911SC in 1979. It consumed a quart every 600 miles, which seemed high for a new car, but PCNA insisted that this was within the normal range for this car and refused to do any work under warranty. He drove the car about 25K miles over a six year period and the oil consumption was about the same when he sold it.

Duke