New guy, ? about wheel size & cert.

Hello all. I've got a couple questions about MB certification and rear wheel size.

First, if I buy a car that's not certified can I get it cert. by MB?

Also, I noticed the rear wheels don't seem to fill out the fenders very well, even with aftermarket wheels, is anyone out there using spacers or are there any pics of wheels out there that fill out the rear?

Thanks