Are you sure the head gasket is leaking? Much more common on the M104 DOHC engine...
This is also a good point.
The M103's rarely leak at the head gasket like the M104's.
The M103's usually leak at the timing chain cover where it meets at the front of the 'c' shaped area of the head. This can be resealed fairly cheaply.
M103's are much more notorious for leaking oil internally through the guides / seals.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".