Recommendations please!
I’m looking at smaller mowers with a 42” deck, since our yard is under an acre. We have a bit of a slope on the side and back yards, so a conventional riding mower is out.
How much of a slope? IMO, a ZTR is no better on a hill than a "conventional" riding mower. If the hill is too steep for a conventional, it'll be the same for a ZTR. The track width may be a bit wider, offering less rollover danger, but mowing crossways on the hill, it's more difficult to keep from "wandering", unless it's a bigger model, and up and down a steep hill you can have issues with a light front end going up, and tendency to slide going down, if you try to stop suddenly, same as with a conventional unless it is 4WD. And like conventionals, maybe even more, smaller ones have a harder ride.
Also, do you ever have a need for doing other than mowing with a lawn machine? A medium size lawn tractor can tow better. I may be a bit biased towards tractor-style mowers, they are what I use and prefer, and I do have a couple of hills steep enough that I get slightly uncomfortable on if the ground is wet, but a ZT would be no advantage. FWIW, I have used a ZTR a few times, but never could warm up to it. They are best for large, open ground where speed (time consumption) is an advantage.
Whichever one you go with, you should investigate what transmission is used on it. Many of the lower end LT and ZT mowers have transmissions not well suited to heavy use on hills.
Overall, I'm a Deere fan and would look first at them, but some of the "specialty" makers (Like Hustler and Bad Boy) that only do zero-turn mowers are worth looking at, too. Toro's commercial line is also a good one, they seem pretty popular around my area. Ferris seems to be top of the heap for ZTR's, coil spring and shocks suspensions, but I don't think you're looking for something at that level.
I mow about 5.5 acres at my home and next-door rental property, and a bit over a half acre at my Mother's house. My three mowers: