I just completed the precess you are embarking on. I currently mow 2 1/2 acres with lots of trees and some hills and knew once I had machine I would start cutting more (neighbor, parents etc.) After researching for about four months, this is what I concluded.
I think any of the big name commercial level ZTRs are good (Scag, Ferris, Exmark, Hustler, Toro, Dixie Chopper, Everide, JD, Kubota, probably a couple more). One of your biggest directives should be a close dealer you like. Driving over an hour for a warranty item would not be fun. Not to mention parts and face to face advice. I liked the Ferris, Scag, Exmark and Kubota. My local Kubota dealer is close but not very helpful especially if your not buying $75,000 tractor. Ferris dealer is extremely nice and only ten minutes away. Scag and Exmark dealer was at the same place, but the one I like is 40 minutes instead of 25 minutes. After talking to local commercial cutters, I was warned even they had poor service problems with the closer dealer.
I then had demo on Ferris and Scag at the dealers. Neither was willing to bring demo to me. This seems to be a regional thing. Around here, dealers will not, but I have read where in other parts of the country they will. The Ferris 3100 rides like it is on air. The 1500 suspension was not as impressive IMHO.
At the Scag/Exmark dealer I rode the Scag. After seeing the two side by side I thought the Scag was a tougher machine. (It is probably like comparing Ford to Chevy, its just what you like.) The Scag was a rough rider, especially compared to the Ferris.
I then went back to the forums and read until I was blind. After a while, the same things kept popping up. Scag is built like a tank and rides like one but has a high quality cut. The Ferris has a nicer ride but questionable cut quality under certain conditions.
Once I found a solution for the rough ride, I knew it was going to be the Scag. A tire pressure between 10-12 PSI (this is what Scag calls for rather than the max tire pressure of 20 PSI) softens the ride a little. The big improvement was a suspension seat like a tractor has.
I am now the proud owner of a 61" Turf Tiger with a 27HP air-cooled Kohler motor. Dealer swapped the suspension seat in for an additional $200. The machine was delivered and a little more than hour later, very tall (had not been cut in over a month), very wet (had just stopped raining) grass was looking better than it ever has.
Hope all my rambling helps. Let us know what you get. By the way, the link to Four Brothers is a good one with good starting prices (most dealers will go down an additional 1-3%).