Do you mean a front wing (maybe fender in the USA), or rear quarter
panel. As Tiger says, the front wing is easy enough (bolt on like
Tiger says), though you must be prepared to address any corrosion you
find on the inner wing/front lower valance when you remove the damaged
outer wing.
You will also have options on replacement wings. There are 'panel
parts', which are copies of genuine Mercedes, but are cheap but not
perfect and will take a bit of 'fettling' (adjustment) to get to
fit/look right. There are genuine new parts, which will be really
expensive, but will fit well, or there are 2nd hand parts, which may
need some minor repair work to the part itself to make perfect. What's
the donor car for the doors like?
If it is the rear quarter you are talking of? If the damage is fairly
localised around the rear wheel arch/lower rear quarter, then you
could get hold of a new 'panel part' repair section. This will as said
require fitting properly, which means cutting and welding. Its a lot
easier than fitting a whole rear quarter though.
Cheers... Rob.