Obviously clearance is the main problem fitting oversized tyres. If you still have the original wheels fitted, measure the clearance between the tyre and suspension struts to establish how much cover you have. That should give you an idea if the sizes you want to fit are feasible.
Another thing to consider it making sure the ET of the new wheel is very similar to the original wheels. Check what your ET's front and rear are and remember that any ET with a smaller number will push the wheel away from the car by the difference in number.
(eg ET 40 original and ET30 new would push you wheel 10mm further out from the car.)
If you have an ET bigger than OEM then you may have to use hubcentric spacers to move the wheel away from the hub of the wheel/tyre may foul the suspension/caliper
Those are pretty big tyres you want to fit there (bigger than my 265x35x18 rears and 245x40x18s on my E55!)
Looking at your other thread and thoughts of supercharging your car.........save your money, sell what you have, and buy a C32/C55 or an older W210 E55. You'll save money in the longrun.