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As I understand it, the .XXX TLDs are pushing for exclusive adult use. Therefore if your site has no adult content then you may be in violation of the terms for that TLD. As Ryan stated, the search engines have no real prefrence to the TLDs, except for in the fact that the users trust them more. Therefore, your CTRs will be higher, which in turn would effect your rankings. When possible .com is the best for most websites. The .co's have a benefit of being shorter, but they have a huge downside for being too similiar to the .com and that they are not yet widely recognized. Put a .co on the side of service truck and the majority of people are bound to think that you simply forgot the 'm' for the .com.

Have you looked into using buddypress at all? Think this will accomplish most of what you are looking to do. http://buddypress.org/ You are able to have forums, blogs, private messaging, groups as well as feature content on a site's homepage. Here is an example of a site on buddypress: http://tastykitchen.com/

Thanks for the tip about HARO! Most of our clients are competing for a very specific set of keywords and we are mainly focused on the success (rank / position) of their keywords. We have followed the recommendations of Matt Cuts related to blogs and articles within those blogs containing natural content that points back to the sites. The article marketing is basically just backlinking the blog articles to get them better PR and more traffic. But your comment creates some good thought about the use of our articles and the energy we put into them.

Thank you for a well thought out answer. I look forward to repaying the favor. We are researching the options you presented and I will post the results here once we know which software we choose.

Buying a aged domain name has numerous problems, not least of which is that if the domain registration information changes and then shortly after the site is up, but completely different to what it had been in the past, Google may wipe any ranking clean and consider it a new site with a new owner. You'll still benefit from any existing inbound links, but it's worth bearing in mind.