1. You need to know CSS (Cascading Style Sheets). Xenforo 2 uses LESS which is a pre-processor for CSS so it helps to learn that as well but not required as CSS itself is all that's truly needed as it works the same in all .LESS files. If your creating a theme work for the style it may require some php or javascript knowledge depending on what's in your themework. A lot of famous style creators use themeworks to help give users the ability to customize the style after they buy it but that much is not required, it only adds to your theme.
2. Not that I know of. Most themes are just templates you edit. Some use the template Extra.less to put their theme code inside which adds any css or LESS to the site and works just fine though a lot of major theme creators such as
@ThemeHouse use separate templates for this which is also possible. However for learning purposes and learning how to code CSS I highly recommend you check out PhpStorm at
https://www.jetbrains.com which is what the Xenforo 2 developers actually use. It has everything from CSS support and LESS support but also includes php support just in case you needed it. You could also use web storm which is the same thing without php support and a tad bit cheaper.
3. Yes the branding removal can be purchased for $250 anytime during or after you buy the license.
Good luck btw! Xenforo 2 is an excellent software as is Xenforo 1. You won't regret buying it if you do. It's well worth the $140 price tag and you get so much use out of a single license that you would definitely appreciate everything it is. The Xenforo 2 resource manager is full of paid and free add ons as well as styles and you would love to check it out.
Also I just made a custom style for my own website in my signature if you want to view it as an example. It doesn't have a themework or anything I just made it with CSS.
