Hi.

New or used car?
After market or high end OE stereo?
I don't know what exactly it is that you want.

But here are both options.

If you just want to upgrade to the high end OE stereo
then have a MB dealer install it and your warranty
should not be affected.

If you're going the after market route, that shouldn't
be a problem either.
Better speakers are available that just drop in the
stock locations.
A good stereo specialty shop can integrate a custom
stereo into your car and make it look stock.
Don't use Best Buy or the likes.
Most important is that quality material and
work practices are used.

A hack job will lead to electrical problems which could
affect your warranty.
But as said before if the shop doing the work does
a quality job then all should be OK.

Do some research on the shop your planning on using
if you're going that route.
Ask to see some of their work on previous cars.
Look for the MECP certification, all good shops should
have it.
Also important is that the sales person is listening to
YOU and not trying to sell you stuff he wants to see sold.
If they start by doing that walk out, they are only interested
in selling you stuff that most likely you don't need.
The sales person also should consult with the installer(s)
and inform him of YOUR wishes and to get his opinion.
Since he has the experience, he can give you alternatives
to what you have in mind, if needed.
Not everything that a customer wants is do-able and if it is,
it might be to expensive.

Louis.