A few sorta-random thoughts follow:
A while back, old-time member
neoprufrok had holograms on his black A4 that he simply could not remove via PC. I dunno if he *ever* got 'em out

But usually it's not *that* tough.
I'd certainly use 4" pads for this.
What product, and how many passes, will depend on how bad the marring (i.e., the carwash marring) is and how hard the clear is (is this ceramiclear?).
I myself would give this a lot of thought if I weren't a) equipped with an ETG, and b) confident that I could either fix it or have it fixed. Consider the (perhaps remote) possibility that one reason why they're selling it is the messed up paint; you don't want to be buying somebody else's serious problem.