The main thing that will affect you between the 26 and the 27.5 is the 27.5 can go a bit further with each pedal stroke, since it has a larger circumference, but it will be a bit slower to accelerate. This could be hidden by gearing between the bikes. These bikes are all about equal, so pick the one you like to ride the most and go with that. There isn't a wrong choice in your list.
You might be able to put 27.5 wheels on it, if they fit, they will change the way the bike feels. It looks like wheelsets start about $250.00 for 27.5. Check the various online retailers to see if they have anything. But if you are interested in the bigger wheel size, you may want to start with the bike that already has them.
I would recommend putting feel of the bike as the number one factor on which one to choose, and everything else as a distant number two. You cn get the coolest best speced bike but if you don't like how it rides you won't ride it.
It's the new fad, so they is what the sales people are going to be pushing. I don't like the feel of the 29ers I have ridden compared to my 26er. Don't have any seat time on a 27.5er, so can't comment on if they are that much better. If you are planning on performing upgrades to the fork or wheels in the next few years the 27.5 will probably be easier to find parts for. But don't compromise on the feel on the bike for a "better" bike.
Hopefully once you get on the bikes it will become apparent which one is the one for you. If not the 27.5 would probably be the one to go for since that is the way the sport is moving.
It's hard to say, the site doesn't list any of the important specs, to determine how it will handle and if off the shelf parts will fit for upgrades. For that price, you may want to look into used bikes, you can often get more bang for your buck.
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