I think the moto's a relatively easy to root, but I can't find anything on XDA, things may have changed.
I think it comes with Marshmallow but Nougat is rolling out, I flashed nougat on my LG G4. I liked it alot, just a shame the rom was a bit crappy and kept crashing.
If you buy it from amazon it shouldn't be locked to any network, and it shouldn't lock to a network after you put in a sim.
Also check out moto maker, you can make your phone personal to you with differenct accents and cases and even a welcome message every time you turn it on at usually no extra cost. https://www.motorola.co.uk/products/motomaker/FLEXR10UK
You can use code UKWELCOME10 to get a tenner off.