What to watch for:
If buying gasser engines, I'd only go Kawasaki on a commercial ZTR. If it has a Briggs, or any one of the several other brand names Briggs uses (e.g. Intek), I'm not even looking at it. I'm having trouble remembering ever seeing any commercial zero turn wearing anything other than a Kawasaki V-twin, they're damn near obligatory / standard.
Heavy cast front casters are a must for me, those things take a serious beating. I look at the folded flat steel casters featured on many ZTR's, and wonder how they're not constantly flexing and bending.
Look at what your local pro's are running, as that's probably a better indicator than any as to how they do under your local conditions and how well they're supported with locally stocked parts, mechanics, etc. Around here, the pros only run two brands today: Deere and Exmark.
(The local pro's used to run a lot of Scag, as well. But it's probably been 15+ years since I've seen any new Scag mower on a landscaper's trailer. Maybe their local dealer went away?)
What to avoid:
Your biggest problem with any ZTR is grass clogging the hydro pump cooling and the engine cooling. But how good or bad a mower design is in these regards is going to be real tough to assess on a dealer's lot.
Once you pick a brand, model, and deck configuration, start reading the lawnsite forum. Lot's of pro landscapers on that forum, and you can search to see if the configuration you've picked is receiving a lot of posts about trouble. On some models, even pro models, you'll find one specific deck configuration (e.g. 72" mulching) has lots of trouble, while other deck configurations are trouble-free.
Other:
The suspension mowers may be worth checking out, especially if your wife might be using it. I run a Deere 757, and they're known for a particularly stiff frame, it really jostles you around pretty hard. The Exmarks are known for a slightly softer ride, even without suspension, but those newer suspension chassis mowers from Ferris and others must be an even nicer ride. I'd not want to ride my Deere ZTR if I had breasts, I think they'd be bouncing off my knees, the ride on that mower is so damn rough at full speed.
The comments about diesels amuse me. We've run the numbers on this more than once, in fact there was a thread debating this just two weeks ago. There is no way any homeowner mowing their own 5 acres can ever justify the high upfront cost of a diesel. Even the commercial guys have to work hard just to break even on the cost of a diesel vs. gasser, based on fuel cost alone. The advantages of diesel are likely more on the convenience of carrying just one fuel for mowers and tractors, and in engine lifespan for those racking up a thousand hours per year. But for any homeowner doing 100 hours per year, your deck is going to rot out long before you exceed the 2000+ hour lifespan of a Kawi V-twin.
