Plugs: don't force them, aluminum heads and all. loosen and spray. sometimes there is a partial thread which protrudes into the CC and when you back them out it drags carbon; anyway you don't want to damage the threads. Mine were hard to get out but I stuck with it and got them out without damage. Just keep loosening, moving back and forth, and spraying.
Anti-freeze: I use the MB dealer's stuff; it isn't too high. I understand it is Zerex G-5. Anything good for all-aluminum engines should be OK. Use good water. My car had the green stuff when I got it, no telling how long it had been in there. All OK here 5 years later.
Brake sensor: you don't have to have it, but it would be nice. there might be a short replacement harness, etc, but I have not had this problem yet.
Dist advance: As I recall the timing is controlled by computer and all the distributor does is distribute. Install it on TDC #1 and 'puter does the rest.
Timing: make sure the crank damper isn't loose (probably isn't) but I don't know how you are determining it is fluctuating: timing light?
Check your cam drive chains for wear: max 7 degrees as I recall. the plastic guide shoe on the driver's side can break and get between the chain and sprocket, causing a big engine wreck you don't want to have.
Hope this helps you until someone more experienced replies.