PR is an indication of "value" and a large part of that is links. If the links to the site start to go away, that will decrease the PR.
But if all the former content is gone, then that should also hurt the site, since it's clear that the former "value" has changed. I don't think there is any clear way to tell what will happen to the PR, but I would never buy a domain based only on the former PR.
Having said that, if you can put a new site on the domain and make it a good one, the effect on PR might be less dramatic just as if the former site had undergone a massive upgrade. This might give you some good advantage over a new domain or one with little or no current PR.