The engine seems pretty bullet proof. Well maintained is good and honestly, I think your PO would have had to have been an idiot to do something that would affect the lifters. Having the correct oil seems to help a lot of this.
Pulleys will also make raspy sounds intermittently...check belt condition and make sure the tensioners are holding belt flat between pulleys.
And to clean things up generally, Techron--the only additive my car has seen.