When in doubt, let somebody else drive. If you think this shop is charging too much (and they probably are), find another shop. Ask yourself if this is really the right time for you to learn how to do this yourself? Just how expensive will it be for you to get it wrong the first time.
I don't work on my wife's car, because she'd kill me if I screwed it up and had to spend the extra time to fix it. My friend Howie works on her car and charges me well for the privledge. I keep my own cars afloat because I can always walk when I make a mistake; it keeps me on my toes.
Have the shop do all the "work", you supply all the parts, including the guides, seals and valves. The guides should be under 10$ each, the valves under 20$. Be sure your supplier is selling OEM guides, not aftermarket. Have you asked these guys (mercedesshop.com) how much they'd charge for the parts?
Let the shop lapp the valves and grind the seats. No doubt, they've already diassembled the head, so you can't install the valves yourself anyway. If you interfere with their job in progress, not only will they not warrentee the work, they probably will refuse to do it. Nobody wants to do half a job, especially if they're being paid. On the other hand, you may not want then to complete the job, that's your right. They'll still charge you for the work done so far. Taking it apart and checking the components is most definitely work and it does take time.
I have it easy, my machinist services all the local dealerships (3 mercedes, several acura, lexus, etc), so he's very well acquainted with the mercedes engines. He knows I'm easy to work with (and almost know what I'm doing), so he gives me the jobber's price 'cause I don't get in his way, don't blame him for my own mistakes and I pay cash.