I built one of the EZ Flyer kits a few years ago with a 912UL 80hp, works great. I also helped Wayne (Blue Yonder Aviation) build a few EZ Flyers, including a twin with a pair of 912ULS, and some Merlins. There are quite a few of both models around here, as well as a Harvard and an Air Cobra look alike. Most Merlins run 4 stroke but one runs an O200 and a couple have 582 2 stroke. The EZ Flyers are probably 50/50 2 stroke and 4 stroke. I believe one Merlin is still used by a civilian search and rescue member.
The aircraft are tough and reliable, easy to inspect and maintain. The EZ Flyer is as named, you would have to be completely incompetent to mess up. I've often made 20-25kt crosswind landings with very little drama, 30kt stall speed helps a lot. The Merlin, being a tail dragger, takes a bit more talent but is still one of the most benign planes up there. I believe it actually has a lower stall speed, something like 25kt.
As far as high altitude and hot performance, I'm a firm believer in the Rotax 4 stroke. All our local strips are above 3000ft and summers can be 80-90F and I've never used more than 300ft to be airborne. My 80hp will easily take me to 8500ft ASL.
Hope this answers some of your questions.