It will take 3 to 6 months to get the content ready so should I purchase a domain name now and stick a page up just to get it indexed or should I wait and launch the whole site when it's finished?
purchase your domain now as long as you already no what domain you would like Google likes older domains, so you can age your domain while working on the content
I recommend purchasing the domain now if you know what you'd like also. In 6 months when your ready to put the content up, the same domain may not be available. Also, it isn't going to change a drastic amount to have your domain age'd 6 months, but it wouldn't hurt. Having a welcome page with something like "content coming June 2011" with a contact form or something would be good also.