If you are replacing the valves, or resurfacing them - do it right. A competent machine shop (one that really knows how to do Mercedes heads) has the tools.

You want a '3 angle grind' on both the valves and seats, with a 1 degree interference angle so the valve will seat at the top first. This ensures a better seal and helps prevent any leaking and overheating. The valve seats are 'stellite'--a special very hard material. The fit of each valve and seat should be checked with 'prussian blue' twice--first to ensure a spot on the valve face contacts the seat all the way around--ensures the seat is ground perfectly square to the valve guide; secondly to ensure a spot on the seat will contact the valve head all the way around--ensures the valve head is ground perfectly square to the stem- and the position of the seating surface (the pretty purple/blue streak all around the seat) on both the seat and valve face. You do not want the sealing surface too close to the edge of the valve or too far down from the head. Similar for the seat sealing surface- should be right in the middle of the 45 degree surface. Only if all these checks are made and passed will the valve job hold up.

Be sure and have them check the valve stems for wear and the guides as well. Each valve stem diameter should be measured at several points to determine any wear. With a new valve, any play in the valve head -side-to side-when the valve is at maximum opening, should be measured with a dial indicator to ascertain any guide wear. Then the fit of the particular valve with the guide must also be checked to ensure the side play is well within specifications. Otherwise, the new valve stem seals will quickly fail and your valve will leak from the get-go. It is easy to drop in a new set of valve guides with the head off.