I have a Toro large frame ZTR (23HP) with a 60" deck and also a Kubota
BX23 with a 54" deck. To me, there is no comparison between the two. The Kubota is a great tractor in every way but the Toro is far superior for mowing. As it should be. A ZTR is a purpose built machine while a tractor is a jack-of-all-trades. I don't have any experience with front mount mowers other then to know they cost more. Good luck with your decision.
A ZTR can beat you up simply because of the speed at which they can mow. If your ground is nice and smooth you will have no problems, if it isn't you will simply need to slow down. You can also whip yourself around from side to side pretty quick if you are too aggressive with the controls.
If you a worried about your back, you can get models with suspension seats. Or even better, Ferris specializes in units with full front and rear wheel suspension.
My advice, get the most power and biggest deck your budget will allow. For deck size also consider the quality of cut a given size deck can provide on your property. A larger deck can't conform to the ground as easily as a smaller deck and so the quality of cut can suffer. A larger deck is also obviously more difficult to work in and around tight spaces. Conversely, a smaller deck means it will take a little longer to cut and it can also make it more difficult to cut along and around obstacles because the deck may not overhang much from the rear tire.
I have found 23HP to be a good match for the 60" deck although if I could do it again I would get the 27HP. There are times in heavy grass or when mowing up hill that I wish I had more power.
As for brand, watch what the professional landscapers in your are are using. Around here it almost exclusively Exmark. I went with Toro because they are very similar machines (Toro owns Exmark), the dealer was close, and I simply liked it better then the rest. I also looked at Exmark, Kubota, Ferris, Grasshoper and Dixe Chopper. I'm sure they are all excellent machines.