Just a note regarding valve guides on these engines, the M104 head design has minimal vave stem/quide side force [ cam-valve swipe] . Mercedes seems to have solved the common M103 valve guide problems with this far superior design and it is not unusual to see them still tight into the 200K figures.
The 104s have a very strong top end....along with some pretty good breath....I would normally recommend to do them on M103 engines just because the 103s have a lot of swipe [side] force compared to engines like the 104s.
So they are just more prone to wear by valve train design....
Having said that, I still redo the valve guides and valve seals simply because the head is off the car and they are cheap.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".