1. Custom fonts are located in /Portal/App_Themes/Default/Fonts, and additional fontface settings lays in _font-face.scss. You can also adjust fontface settings directly in Default.css, but it's not recommended if your planning to use sass later (it will be overwritten on sass compile). Instructions about using fontface you can find here
2. Yes you can, but again, it's not recommended if your planning to use sass later.
3. For using sass you must install all prerequirements. Instructions about that you can find here
Hope my answer will help you!