Knowing lots of M103/102 owners, and being an ex-M103/102 owner myself, I would comment that they have a lower incidence of head gasket failure than the M104 engine.
However, the front cover is quite failure prone. Follow Gilly's advice on that repair, and it'll last much longer than it did from the factory.
I've seen some M103 engines with leaking main seals, front and rear. On further inspection, found severly overfilled crankcases.
Overall, I wouldn't characterize the M103 engine as a "leaker." Sure, they can, and do, but I think overall the 103 is pretty good.
1998 C230 "Black Betty"