where should i look for the dreaded rot?

i went to see a car yesterday and possibly going back today with a view to have a test drive and give it a second, thorough look over.

i checked the sills, rear chassis legs, near rear bumper and spring mounts. the bottoms of all the doors and the arches and all seems fine

The car still has what seems to be all original panels, and hasnt been welded underneath which is good, but are there any other places to check that i may have overlooked, or places that need particularly close scrutiny?

Also when i take it out for a test drive, what sort of things should i be looking for? i can check that it drives in a straight line, any odd gearbox noises etc, but what else.

when i started the engine it had a slight tapping noise, which went away after 1min or so? is this normal. what should i look for under the bonnet? common leaks etc, signs of wear and tear.

sorry for all the questions but just about to take my first step in to cosworth territory, and wanna make sure i start off with a good car, as this one is nice, but its also top money

any advice appreciated

cheers

rory