Someone from the US or Oz mentioned that they had used the same source as me - a UK ebay shop, I think called 'the gasket shop' based in Yorkshire - I think Leeds or Bradford but it might be another city. They have several options, gasket on it's own, or various kits including valve seals and/or exhaust gasket and/or intake gasket and/or coolant sensor seals etc. The gaskets themselves are Elring, which I believe is what MB puts in at the factory.
Head bolts are high torque and do stretch, however there is a specification for the stretch (I found mine in Haynes but I don't think it covers NAFTA engines) so it is normal to measure the bolts and if they are still within spec re-use them. You can of course just get a set anyway if you can't find the spec or want err on the cautions side. I think Mercedes always use bolts that need 12 point spline bits to undo (I think mine needed size 10, or M10) you will need some long ones, probably 4" and 6" - don't mess around with a short one on an extension bar, if it comes off you might lose it into an oil gallery (took me ages to fish one out, thank goodness it only went laterally and not down). I bought a big set of various lengths and sizes in the end.
Make sure you use strips of rags and a screwdriver to completely dry any oil or water from every bolt hole, apparently hydraulic action if you try to put a bolt in a hole with oil or water in can be enough to crack the block.
If you know it is definitely the gasket and not the head, you may be able to remove the head with injectors in place - I don't know if you have direct or pre-chamber injection, if direct make sure you have some wood blocks to place the head on so the injector nozzles can dangle in fresh air and not get mashed on the bench.
My manual says to remove the head with exhaust manifold in place and unbolt the latter once the head is off, it is poorly balanced like this but much easier to get to the manifold nuts, and again, if you don;t actually need to remove them manifold you could just leave it on (it might be enough to hold the injector nozzles clear of the bench, it was on mine).
Good luck, it is more daunting that it is actually hard - I have had mine off twice now!