No, you cannot change the wheels, no, I am sorry. The best all-terrain, like true all-terrain, is of course not just one - it depends on the parents, but for just all-terrain use, a three-wheeler is your best bet. For all-terrain, it is actually better to have the child lower for the pushchair to be stable. I personally would think of a Baby Jogger or a Mountain Buggy.
From reversible models, the older Bugaboo Buffalo is the best from my point of view. The Trailz is also a nice choice but I, a tall mom, found it harder to get up curbs.
You can, indeed, swap canopies on the Fox line. Most Bugaboos have their canopy lines to mix and match.
I would definitely recommend a second hand stroller, YES! I owned several of them. The danger is to check before and have to believe the seller, but to get a pre-owned stroller is definitely economical as well as ecological, so if you can go check the buggy out, or believe the seller, it is a great choice to get a better model for less money.
To your last part, the Surf is FANTASTIC to push but has a very very small seat. It is also quite bulky for what it is. The Joolz Day, another fantastic-to-push pram, is also bulky (very!) for a mostly smooth-terrain model, and even if I like the seat more than the Surf's, I would not go for the day simply because of the price-performance ratio. The Fox is definitely the lightest, roomy enough, and very very nice value for most terrains except extremes, of course. That would be my best choice, but yeah, it is not small after folding. And Stokke, there, the Xplory is VERY large after folding (again) for just a city stroller. The high seat is awesome, but I would not get this one as it rattles on bumps, moms usually get it to show off in the malls in my experience. If a Stokke, than either the lighter urban Beat for foldability, or the all-terrain Trailz for sure.