timing chain, valve guides, some of the stems, the timing chain actually snapped just prior to me buying the car so the shop fixed it before I bought. only reason I got this deal is my family had used the place for many years. I no longer use them though so that won't help very much. there is still a little oil in the coolant because it was not adequately removed, but the condition has not worsened and there is no evidence of leakage other than a slight slick feeling on the front of the head cover, but my dipstick stays just about near the full mark. I might be using a pint every 3,000 miles which is essentially nil.
I would argue that M103s are more reliable than M104s and M104s have their own leakage problems. Many I6 designs have it (not sure why they seem to have it worse than V6s). M103s are good engines, 300SEs are fairly underpowered but that is why there is 560SEL, S500, and S600 ::snickers::
currently just 1989 300SE 185k miles