My initial thought was "brave" but you might be OK.
Only a theory of mine but my guess is a lot of S320 petrols are cared for probably as most bought were CDIs back then.
Whoever bought these, despite them not being great on the juice, probably wanted more from a car than just economy or performance (S500 / S55 etc.).
So unless it fell into the hands of someone who didn't care for it, you might be OK.
Saying that, I doubt anyone looking to skimp would buy one second hand so it could well have been loved by most if not all owners.
It's not done a huge mileage (if genuine) and I'm guessing you bought on condition and history.
So you are probably just up against Mercedes 2000-era build quality and I do wish you a lot of luck with that but hopefully by now, it's just ageing that will be the cause of expenditure rather than premature failure!