I tried to pinpoint all the changes from the 1st to the third generation in the descriptions of each model. In short, the original, Fox 1 is the first one, and then, upgrades were coming - the Fox 2 was pretty much made easier by simplifying features (like easier removing the fabrics), and the third iteration, Fox 3, had multiple smaller changes (like wheel design) but also more padding and added the bassinet ventilation/panorama window.
There is no bad choosing a Fox, it just depends on whether you want a new one with a warranty and the cooler bassinet, or you find a good one pre-owned, then a first or a second is also just fine. Plastics and longevity-wise, newer is, of course, usually better.
About the Buffalo, it only is worth getting if you don't mind the a bit bulkier even fold, and a bit heavier whole - however, it will also mean the ride over terrain will be much better. About the sizing, the only difference really is that the Fox is lighter and a bit smaller than the Buffalo. I cannot think of a solution for your back door, only a very narrow stroller which will take away from other features. Taking the wheel off, or folding the stroller is the usual approach. I don't know, however, how narrow is your door. If it is just slightly narrower than the normal 80 cm, it should fit still, the Fox is 60 cm wide.
About the Stokke Trailz, it is a very high-positioned, sturdy stroller, but I am afraid it wasn't better to Fox/Buffalo from my point of view - it felt too heavy, too bulky (much bulkier than BGB) and for me, the center of gravity was a bit off, making it hard to pop the stroller on a curb or a higher bump. Definitely a quality, longlasting one, just not as easy to use.