You're right, the M103 motors are known to use a bit of oil.
Valve stem seals are a common problem; I'll assume the tech who did the work knows what's he's doing. There's a U-shaped gasket at the front of the motor that's known to leak, but the consumption you describe is high and if this item were the cause you would likely see an oily mess in that area. There should be a large plastic encapsulation panel underneath the motor. On your vehicle, there are 4 small sheet-metal type screws with a hex head holding the panel on. An 8mm socket gets them off. You can access this panel from underneath without lifting the front of the car. The back two bolts take a bit of a stretch to reach, but it's not difficult.
With this panel off, you might get a better idea about leakage. If it's wet, I'd clean the panel, reinstall and drive for a couple of hundred miles; then pull it again. This "might" give indication as to where leakage is coming from.
The rear corner of the cyl. head - passenger side is another area where leakage is common on M103 motors. If you see this going on, your head gasket needs changing. Normally you'll see oil on the garage floor in this situation, but it's worth looking at anyway.
MB upgraded the valve guides for the M103 motor in April of 1988, but you give hint as to what might be your problem - 80K miles. That's about 6,000 miles a yr. and I'd bet that a previous owner(s) had the car sit for who-knows-how long. Cyl. heads can dry out, better guides or not; this may be a big part of your problem.
There's always the valve cover gasket and the gasket on the oil filler cap - littles things add up.
Last but not least are the piston rings, but with that kind of consumption I'd expect to see some smoke.
In the end, I believe you'll need to find a qualified MB tech - not the "occasional" variety, but one who's spent many years working the motor you have in your 300.