The 300E was one of the most popular models Mercedes ever produced. I'd have to say over 50% of the service work I do is on this model. In my opinion it is one of the most durable and reliable model/engine combinations of any car manufacturer. I myself have owned one and driven it over 200,000 miles with very few problems.
The valve stem seal problems were truly valve guide problems that Mercedes has attempted to correct with new/improved valve stem seals. This never worked; the only true way to repair the M103 engine with an oil consumption problem due to valve train problems is to replace all the valve guides - a.k.a valve job. This typically runs between $1,500 and $2,300 and is a MUST if this engine has not had this repair.
Definitely have this car inspected by a MERCEDES-BENZ technician. If the car is in good shape and needs minor repair, my thought is to buy it ASAP. If not, keep looking. The good ones are out there.