The real value in having a PR 5 site is the power of the backlinks you can give out.
A pr 5 site itself may or may not actually rank for very many keywords. The ranking of a website depends a lot of the domain authority of that website, as opopsed to the page rank. The page rank depends to a very large degree to the number of links pointing to it (though not as much as most people think it does).
For instance, suppose you have a series of sites targeting the keyword "Donate to the Democratic Party"
Politicians at the local level, state level, and national level could target this keyword. Within any of these sites the may be a PR5 page.
But even if a local politician, a state politician, and the president of the US had a PR 5 page on their sites, these pages in each site will not have anywhere near the same authority.
My local politician page is simply not going to outrank the Presidents page, even when they are both PR5.
As for keeping that PR, if the content does not change with the owner, the PR should not change.
If the domain name is all that is being bought (so the content changes), the PR will fall away, because the links will be less relevant.