The valve clatter is likey a lifter in the early stages of going bad. I wouldn't mess with it until it is a constant clatter. Using a good synthetic such as the Mobil, it may never get worse, but if it does the lifter is a replaceable part of the rocker arm.

As far as the best way to get an idea of the general condition of an engine, I personally would remove the valve cover and inspect the cam(s) The cam lobes are the highest point of friction in an engine and when an engine's maintennance is neglected, the cam is the first place to show it. If the lobes show lots of "streaking" it could mean your 2.6 is a ticking timebomb