For what it's worth....
Having seen Euro Car Parts delivering on more than one occassion to a Merc specialist garage, guess where I'm now getting my stuff? That and the local motorfactor for good quality oils and the like.
On the tyre front, I usually buy new ones just coz it's hard to find large size s/hand and I pay mid to upper range (at least Euro/Japanese branded and Conti's etc if similar price which they usually are) for the AMG but shop around - had cheap tyres on other performance cars before but things like wall stiffness and feel just ain't the same. In fact, I DID get rid of new cheap on an Alfa I bought and fitted Pirelli P6's to my absolute joy. I do get s/hand components when the price difference is mad and if it fries again early so be it, but so far so good.
I don't know if it's the same as m/cycle world but high performance and grippy tyres definitely lose some of thier sticky stuff as they get older and wet performance is the first to go, as I discovered after slithering down the road on tyres a few years old. Just makes you think...