Nice car looks pretty clean for the most part. Interior is immaculate!

Regarding the rust, from what I'm learning on my 98 E300, there's most likely rust behind the outer skin of the rocker. There's an inside rocker as well. Both can be repaired and structurally hold up. Rust can always be fixed. Just cut the metal and weld it, fiberglass is NO way to fix rust. If you repaint the car, try to have an integrated clear coat and base coat done. Less clear coat issues in the FL sun.

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k