Not being a master mechanic...
1. Now that they are off (a lot of labor), I'd take them or send them to a shop that knows how to do aluminum MB heads. I have the shop manual and there is a lot more voodoo and care involved then on, say, an old SB chevy head. According to what I've read, Liquid nitrogen is involved in installing the valve guides, fer goodness sakes.
I don't mean to scare you, but the shop manual says that helicoiling the block should be done when replacing the heads (no doubt, because the block is aluminum). The famous Road and Track article quotes an experienced mechanic who claims that with care, this is not required. Needless to say, make sure you have a good and accurate torque wrench. The gaskets aren't cheap, and you want to do this only once.
See this search:
560 SL cyl head gasket, heli coil necessary
If you do it, be aware that there is a MB guide tool for this - the drill holes for the helicoils should be drilled one degree off, and this guide plate has this angle built in. Again, do you need to do this? I can't say.
2. Given my experience, your motor mounts have GOT to be shot. My car, 3 years newer and 1/2 the miles (with a more powerful engine, admittedly) and an apparent creampuff with 2 women PO's had one smashed and one broken. It's extremely difficult to see their condition without removing them, unless perhaps somehow measurements against an identical engine/car with a known good set could be taken with a ruler. I would do this especially because I think it contributes to the infamous "broken radiator neck" problem - the engine pulls on the radiator hose because it's sitting loose.
On the chain, my 0.02: Depending on what you want to spend, I'd do in this order:
1. The top chain guides
2. the tensioner
3. The tensioner arm
4. The chain
You do already have everything apart...
Eventually at your mileage and driving rate, you are going to have to deal with the lower guides under the timing cover, which is a major job (as opposed to what you are doing...heh heh).