Valve seals yes.....but valve guides with the head in place? I don't think so.

Concur. I've replaced guide seals like this...it can be done....you pressurize the cylinder with air to hold the valve in place, then using a tool that depresses the spring you can get at the keepers and seal. But the guide itself? Don't believe so.

I was faced with this decision myself with a burner recently (kids car). The problem is that if the seals are brittle and losing their ability to seal, (they are an elastomer that don't survive past 100-150K miles due to the heat mainly) the guides themselves may be so worn, that the seals are only part of the problem. You could change the seals, but what can happen is that the movement of the valve in the guide bore (if worn) can allow oil past the seal anyway. Depends on the clearance. Quite a gamble. Not worth the trouble or money..besides, if your quote includes guides and seals, it is a good one. I would never change guide seals alone, unless I knew for sure the guide history and that it were certain to solve the problem. Anyway....if they install new guides...it makes sense to replace the valves at the time....I don't know how expensive MB valves are, but while the head was off I would just have the valves replaced and your standard "valve job" done. Especially if the car is a keeper.

I just had my sons 4 cylinder apart for this reason, and just replaced the guides, valves and seals. My philosophy is: If the head is off, might as well use the opportunity....I don't want to repeat the exercise.

dave