1. ClassiPress is a WordPress theme, so you need to have WordPress installed in order to use it. So yes, it would be advantageous to learn how to use WordPress itself as this is a big part of the general functionality and how you will administer your ClassiPress site. Like anything though, practice makes perfect ;)
2. You won't be able to read the full document as a non-customer, but we have a step by step installation/setup guide for ClassiPress
here, and of course our support forums is where we are happy to answer your questions if you have any issues.
3. Yes, all theme purchases currently include free lifetime updates.
4. No, it is a one-off cost.
5. The .psd files are useful for web designers who wish to redesign the layout of the site, but if you don't have any knowledge of css/html then you would probably not use them (unless you're hiring someone to redesign it for you)..