A Sigil relevant Poll?
Sigil has no control (or license to include fonts) of what fonts you choose.
Sigil allows any 'proper' stylesheet. Go for it
Files,fonts are added via right click in the book browser.
IMHO adding images: it is easier to place the Image into the page using the Image place tool. (the image file will be imported to the Images section).
If you add via the Book Browser, then you need to hand code the points of usage.
Sigil has no provision for an 'inline' Contents page.
Sigil 3.x builds a TOC into the NCX file from h# tags (with a provision to hide some tag from the TOC (press f7 and untick)