I've purchased dropped domains with PR1/PR2 and the domains retained the Pagerank. I believe that a dropped domain with keep the original pagerank in so far as the domain retains its backlinks. Of course, you will loose all the deeplinks unless you figure out a way to replicate the sites page structure.
There's really no guarantee of anything if you are buying a domain and using pagerank to place a value on it. It's too easy for someone to buy links or link in from their own site or friends. So in this kind of case, you aren't getting anything.
Rather than worrying about the PageRank, you should analyze the backlinks, traffic and search rankings.
Thanks for the information! That is what I am currently doing.