Looks a decent car although would need checking carefully as I'm not convinced all the parts are factory fit as described. I think the rectangular tailpipes are wrong for the year ... the blacked-out grille, aftermarket window tints and AMG badge are definitely not original!
Looks nice but yeah rear cans should be the oval variety and the facelift Amg kit never had the bootlid spoiler either so its a bit mish mash but easily rectified at a price.
Standard R129 stuff (there are plenty of threads on this) - everything electrical, especially the hood/rollbar/windows. SL600 specific ... any suspicions whatsoever about the engine (misfire, noise) I would walk. All SL600s have ADS (adaptive damping) so vital you check that all works perfectly, at that age & mileage you need to see receipts for replacement of the accumulators (spheres) or factor in £1000 or so for that. It should raise & lower smoothly on the rocker switch, sport/normal mode should work, no warning lights, no visible leaks etc., ride should be superb. Any sign of bouncing/hardness points to failed accumulators.
My concern would be that someone bending the truth about things like the exhaust and bodykit might be hiding other stuff. The car could be fantastic, but I would check it over very carefully. Make sure the alloys are genuine (they look ok in the pics but hard to be sure) - replicas are very common and are OK but the real ones are worth a lot of money so often get sold off separately.
Best thing to do is get the VIN, decode and see if all those AMG bits are in the option codes.
Code 772 - AMG styling and Code 793 - AMG alloys should come up if true as per the Ad says.
Saying that, not a bad thing if fitted afterwards though, so long as done properly and that you're happy without proof.