I would not let the valve seals alone be the deciding factor in looking for a car. If you find a nice example of the M103 engined cars, the valve seal replacement alone is not a deal killer.
If they have not been replaced, it would be a few hundred dollars to accomplish this. A tech can replace them without removing the head.
If you are leaking oil from the most common place on an M103 engine, which is the upper camshaft cover, this is not a difficult thing to fix if you use the right sealant. MB sells a sealant for this that is translucent blue. If you remove the dist. cap, rotor and rotor piece, you unbolt and remove the upper camshaft cover quite easily. If you THOROUGHLY clean the groove that the lower seal fits into, then goop everything below and above that seal, then slide the cover into place being careful that the new shaft seal goes in place with oil on shaft and seal, you can seal this with no problem. This blue translucent goo allows the cover to slide on the lower seal.
'01 C Class, Six Speed
'84 Euro 240D, manual, 533K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in