Via a friend I've found a little website that is up for sale and I like it. All the obvious things have been checked and confirmed (legit, respected, traffic is certified and so forth).
I've also started looking into the SW itself (a sort of heavily customised drupal but no changes in core files so not a fork).
What else would you investigate? Not so much from a business point of view but from... any other point of view :)
check info:domain.com in google actually returns the site (if people are selling a site and faking the pagerank (some do...) or site index, a info:domain.com won't show the actual domain.