Conventional wisdom: Dust inside is not a big issue unless it is very, very thick.
Dust...or any stuck-on debris or scratches or chips in the glass... on the outer front element surface is likely to be seen in the photograph.
My 16/3.5 Fisheye-Nikkor had obvious smears and condensation deposits on its internal rotating filters** which sit in the middle of the lens, but appeared to have no effect on the image formed by the lens. Of course, since I could see the junk on the filters if I held a bright LED torch up to the lens, I wasn't happy until I had the deposits removed by service personnel.
** The lens you are considering does not have internal filters, so one less thing to go wrong.