All mercs rust sadly,but it's not major stuff at that age.
I know earlier clk's and e classes had spring hangers literally rot out from under the arches but I think at 2003 you should be fine.
The lump is pretty infallible and quick too.
Mercs in general have had a few electrical gremlins and poor switchgear compared to those of old but their still great cars.
Sl500's were always a well made motor being the flagship models.