I think the 16" would be fine. I make hats all the time and that is what I use. I cast on 80 for reg and 88 for large.
I've been making chemo caps and I use all thicknesses of yarn and so far the 16" has worked with them all.
I'm in the same boat as you when it comes to finding circs. There are 3 places in town, Hobby Lobby, JoAnns and the LYS. If I want to make something, I better plan WAY ahead and start looking for circs, otherwise I just sit and stew. I'm about ready to buy interchanagables, but I'm not sure about which ones to buy - too many opinions on which is best.