1. For Joomla 1.5 and 2.5 versions are the most stable ones - whatever version you use does not matter - the both are very stable and you will have no problems creating user friendly and SEO optimized site!
2. It depends on you - Quickstart is easier, but you can play around with both of the options - install quickstart and maybe on another sub-domain the template itself. Play around with it and see the difference in both!
3. It is good to know the basics for Joomla because CSS and PHP are huge so more time is needed top learn them