What are you filming, should be the first question.
Just go ahead and go with a few pieces of Vintage glass. Don't go with an ultrawide lens like the 11-16mm unless you know how to use normal lenses first. I laugh whenever I see some kid filming with a tokina at 15 or 16mm because they can't keep up with their subject. The kit lens offers a solid FOV at 18mm, regardless of the distortion.
get a 28mm/30mm/35mm prime for a standard lens, and maybe a 50 or an 85 for portraits. Your kit lens is mostly fine for the 18mm focal length.
If you want autofocus for photographic purposes, look into a 24mm or 28mm f2.8, or if you'd rather get a zoom, buy a tamron 17-50, since with video, you won't be able to outresolve the inconsistencies in the glass anyways. Either way you're looking at 200 or 300$
Have a nice day, Bluename.