It's late and no one's chimed in yet with the most helpful advice, so I will. You need to use the search tool. Search for "300sdl" and "603", among others, in the Diesel Forum. You can also search for postings by "GSXR".

There's a wealth of good information to help you make an informed decision.
The mileage wouldn't necessarily scare me away. Hopefully the seller has all the records. If so, that's a tremendous plus to help you understand what has already been done on the vehicle.

Good luck. IMO, there's nothing better than a well maintained 603 (engine number).

__________________
Chris

'04 ML500 - 53k, Inspiration Edition, Desert Silver
'11 Audi A4 Avant - Brilliant Black
'87 300SDL sold
'99 C280 Sport sold
'85 190E 2.3 sold