Welcome to the Q&A Forum

Mike's suggestion about understanding what the current value is for that site and all it's individual pages.  If there had been significant value at one time and it's dropped off, you'll need to rebuild it all over again.  Also, if you're buying old domains and replacing the content with all different content, you're essentially starting from scratch. Domain names by themselves won't provide enough value to make it worthwhile without all that new from-scratch work, and just using them for redirects is definitely not a quality SEO tactic in 2011.