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Two great answers here already. Basically, you may have a strong resource to work with but Google is also keen on not allowing brand new site owners from benefitting too much from the authority of newly-purchased websites. Google is a domain registrar for the sole purpose of accessing comprehensive whois information and understanding when websites / domains change hands. If a domain's registration information, hosting and content changes drastically, there is nothing stopping Google from removing the authority that domain once had. Again, this is not to say that the domain won't be useful, but you will probably need to build a useful site on that domain that reflects the quality of the links that have helped it achieve a PR6 in the past. Simply redirecting it to a new location will probably not be of much use - I seem to remember that Google started stamping out that practice's usefulness five or six years ago. $100 also seems quite cheap for a PR6 domain, but maybe you lucked out!

Actually, you are correct. Google's PageRank algorithm has always been all about links. Check it - PageRank works by counting the number and quality of links to a page to determine a rough estimate of how important the website is. The underlying assumption is that more important websites are likely to receive more links from other websites. More on http://en.wikipedia.org/wiki/PageRank. This being said, Google's toolbar PR is not nearly as influential as a ranking factor as it has been in years past.

not for PageRank, because as I stated, page rank is only links, but for relevancy yes, it wont be relevant. You could also be importing problems with the domain along wit the PR,