The biggest problem you face is that there is no tool out there that really reflects accurately the Google PR - the toolbar is known to be faked and has been for two years, any smart webmaster knows how to get fake PR for a domain using 302s and so far no one else has been able to hack Google well enough to get into their real PR database
Im glad you decided that PR is not a good indicator of trade worthiness - something Ive been preaching for years as has at least one other LL owner here.
One indicator you can usually trust is Googles Directory (if the site is listed there) as they tend to update that quarterly with a good indicator of the pages ranking and it also means that the page is listed in DMOZ(for what thats worth)
Overall - looking at a site and its history using domaintools (the new whois.sc) and what type of linking they do - as well as the most important part - does the site add any value to your surfers? - is the best approach
Take a look also at whether the site has any history of being listed consistently in Googles SERPs over time - dont use a one day snapshot as its really too easy to play for a few weeks and then drop completely.