Well I haven't read all 3 pages of thie thread, but my thoughts are:
1) Definitely on a car over a few years old it's ok to have a self-service history, I only take mine to S2 cos they don't charge much more than the parts would cost me anyway, so I let them deal with the mess. I always did mine before, and just kept a sheet in my car file to show dates of what I'd done, with receipts for parts and oil.
2) The tools you need last forever and are cheap - my plug remover was £10, and oil capture bin £5. You empty it down the tip, and take it home to use again. Stick with djames's advice re the oil filter - I only once ever had trouble removing it by hand (heat being the main problem) and a screwdriver jammed in makes it easy to remove.
3) I usualy flush the engine every couple of years on the oil change, using some cheap thin oil by Comma that I found for £10 a gallon. I've heard the risks involved... but seriously if it's deposits that are holding your engine together, you've got problems beyond needing to change the oil
So yeah, I wouldn't worry about doing it, and I DEFINITELY wouldn't pay MB £400 to do it.