I'm looking around for a domain to buy that has age and maybe even PR, and I notice that most of them have a parked page filled with advertising on them, but if I want to develop a new site then within a short period of time the search engine spiders will see three completely different sites. The original site, then the parked page with ad links, and then a new site with it's own site structure.

So it begs the question how do you develop a new site on an old doamin without losing pagerank? Should I be using Google's cache to recreate as many of the previous pages (not their copyright, just their keywords), or should I 301 the old urls to the new pages with similar keywords, or just forget about trying to keep the SEs happy and develop the new site.

Also, how quickly after buying an existing domain does the new site need to be developed?

And, what is the optimum amount of content that needs to be added to the new site when it goes live to prevent loss of pagerank.

Hope all this makes sense.