If you plan on keeping the car a few more years new valve seals would be a good investment; 103 engine seal materials changed in mid-'88 (viton for BOTH inlet and exhaust). The original seals are likely hardened and cracked and cannot be "revived" with any motor oil or snake oil crankcase additive.
I think others have reported that the job costs around $300 at a commercial shop, which is not unreasonable. There are likely aftermarket seals available, but it's tough to know if they are as good as OE. I'd recommend insisting on the MB parts to make sure you get the best quality seals.
Excess oil consumption will eventually reduce the effectiveness of the catalytic converters, which could lead to failed emission tests and the need for new converters, and given their cost, a valve seal job is worth the insurance.