-Buying the current version and Apple releasing a newer, faster, better one is always a concern, especially after the initial release of a product (iPad example - I got a 64GB one, and soon after, they had the iPad 2) However, since this is a mature product, if this your greatest fear, get it at the end of October. By then, it will either be updated (iPad mini 5) or discontinued.
If it is discontinued, they'll be much cheaper, as no stores will want the inventory.
-Another person answered your IOS 11 question. Apparently it works well on it. (whew!)
- I look at the mini and the 9.7/10.5 and the 12.9 as separate items with a common denominator. One may be great for one person, and not work out so well for another. I wanted a 12.9in one for flying, and someone said that it won't fit in the flight deck of a 777. Well, I went out to a 777 that we had handy with a cardboard cutout of a 12.9in iPad, and it did fit, but it covered up too much of the other non-essential items, like the window, radios, engine levers, and wouldn't fit on the yoke. Oh well... A 9.7 or 10.5 worked fine, but I'm not rated on a 777. Still working on the Cessna 172 rating. (note: if you're a smart-aleck to someone that works in a factory that makes airplanes, they can give it right back... )
So, my advice to you is to take a look at what you want/need, and go from there.