If you want to do it yourself, this thread will be your bible
http://www.mbclub.co.uk/forums/engine/49516-howto-change-head-gasket-12v-3ltr-m103.html.
I did mine as well here:
http://www.peachparts.com/shopforum/showthread.php?t=241701
I HIGHLY recommend removing the intake manifold and lift the head with the intake detached. It saves a TON of time and you have less chance of making a mistake because you don't need to remove anything from the intake side other than the bolts. Its a little more fiddly, but on refitting, you will thank yourself so much. Also if the water pump is still the original one, I would say replace it now. Get the head completely rebuilt -- Valve guides + seals. Should cost about $350 inc labor to skim it. You need to pressure test + skim the head anyway, so while its at the shop, get them to do the valve guides + seals.