If you are buying it some an individual and not a company, you usually get the content. You would have to confirm this with the seller. If you do not get the content, you can keep the PR as long as you quickly add content and keep adding content so the page rank stays. I've done this and on some my page rank went up and the rest it remained the same.

What you should do is get the domain, do a backlink check on it and see to which pages they backlink to, then go to Google cache of that page and recreate it and put it in the same URL it was before.

If you buy the domain AND get the content, you should retain all PR.

If you buy the domain and do get the content, then you'd better get a site up fast. Before them taking the content down, I would crawl it with something like Xenu Sleuth to at least get a list of ALL of their old URLs and corresponding title element values. If possible, I'd even scrape the site just to have as a reference. I'd get new pages up as fast as possible and redirect all old URLs to the URLs on the new site whose content most closely resembles that of the old pages.

It depends entirely on the seller whether you get the content or not. If you have bought the domain only but decide to copy the original content, then you will be breaking copyright.

You should really be looking for complete websites for sale rather than domains for sale.

If you're buying a domain you don't have rights to the content. That's a separate deal, however you could download all the pages and have some copywriters rewrite it so it's original.

it will lost its PR in next updates if you remove content of it.