A 300D is highly unlikely to be something you'd ever see at, say, Barrett-Jackson, so don't be concerned too much about decreasing value (which is probably at the low point in the curve anyhow). A properly done paint job in tasteful colors (i.e. no ghetto or ricer colors) will increase the value, as would tasteful mods.

I've gotten the impression that many denizens of this Mercedes Benz forum take a dim view of any sort of mods, with reasoning along the lines of "Mercedes didn't design it that way and they know best" or "That isn't traditional" -- I'd expect you'll stir up a hornet's nest if you discuss mods. Heck, some folks are against fuel additives like Diesel Kleen because Mercedes didn't endorse them 25 years ago! Time, products, and situations change and it's appropriate to adjust things to meet the times.

FWIW, I got some flak when I mentioned I'd be replacing my 300td's roof rack with a Yakima and replacing the fog lights with H1 high beams.