I bought a 2000 E55 AMG a couple of years ago and it's been rock solid with the exception of the catalytic converter on the drivers side. Only $2500.00 from MB and no after market replacement available. I was lucky enough to have a friend at MB that managed to get it replaced under warranty.
If you've searched the MB forums you already know about the common problem areas such as cat's, crank pulleys, ball joints, ac evaporators etc. Parts are expensive on some models but there is are strong after market sources such as Pelican Parts!
The rust issue is most common on the top of the door frames and at the trunk lock button/cylinder. If you pull back the door seal at the top of the frame you may find corrosion. There was/is a factory campaign where they spray some wax in the seal channel to displace any moisture.
As with any high end vehicle many fall victim to "deferred maintenance". Lots of second owners finance the purchase and cannot address the necessary repairs and maintenance.
You may get lucky and find a gem! I love mine but paid considerably more for the extra hp and handling.