Well, I don't own a ZTR but have been looking recently. I am limiting my search to commercial grade mowers so my comments will be based on those.
I've spent hours reading forums to get a handle on the various brands. I would recommend that you spend some time at LawnSite.com
Like CUTs, each brand has its loyal owners and I'm beginning to think that any of the major brands would be fine if you have a good dealer. I've looked at ExMark, Scag, Hustler and Kubota. There are others that have good reputations but I haven't seen them in my area. You will get anecdotal reports on each brand that both praise and crucify them. Other than Kubota, they all seem to use both Kawasaki and Kohler engines on various models and some have a Honda engine option (at least Hustler does). The only diesel model I've looked at was the Kubota. I've read that Grasshopper uses Kubota engines in their diesel model, but I'm not sure about the others. You mention long life as a factor. I think a diesel would certainly give you the longest engine life. But if the rest of the mower konks out on you then , well, you're just left with a diesel engine. Of course, the diesels are going to cost $2K to $3K more than comparable gas version. You will get better fuel mileage with the diesel, but around here diesel now costs more than gasoline.
ZTRs can be fast and really reduce mowing time. Some manufacturers state you can mow 4 or 6 acres per hour. However, high speeds on bumpy ground might translate into internal organ damage to the operator /forums/images/graemlins/grin.gif . Therefore, one thing I've learned is to be sure you check out the seat and look for the option of some sort of seat suspension to soften the ride. Again, opinions vary and you should demo several models to see which one feels best to your posterior.
I would also determine what size deck you want. Do you have any fence gates you will need to pass through? I have been advised that the 44"-48" decks will yield a better result if the land is uneven. Less scalping. Of course, a larger deck (52" or 60") would reduce your mowing time due to the larger swath from each pass. The HP requirement would depend, at least in part, on what size deck you get. I would get the largest HP engine offered for whatever deck size you decide upon.
Another thing to determine is how you will manage your clippings. Will you bag, mulch or side discharge? The various manufacturers have different designs for their mulching decks, for example. Also, I've read that there are differences in how a particular mower handles wet grass or long grass. Some complain about excessive clumping if the grass is too long or wet at the time of cutting. It seems as though there are differences in how they handle slopes. Hustler, for example, states their mowers are better on slopes than the competition.
Regarding maintenance, if you will be doing this yourself, then things like the number of and accessibility of the grease zerks would be important. I've read that Kubota has several and at least one is a real bear to get to. On the other hand, the Kubotas have a built-in jack on the front that allows the front end to be raised for easy access to the underside for blade changes, deck cleaning, etc.
Well, I think I've gone on long enough without really helping you. In the end, if I do purchase a ZTR, I'm leaning towards a Scag. They enjoy a good reputation with the professional landscapers, the dealer is very close to home and my experience with their mechanics has been nothing short of outstanding. I can't say I like the Scag any better than the others (in fact, if I had a nearby and decent Hustler dealer, I would probably by a Hustler due to the apparent ruggedness and value) but I think it will do everything I need and will probably last my lifetime of residential use and I have a nearby dealer who I know I can count on for good service (but, sadly, not necessarily the lowest price).
Good luck.
Edit: Well I see Mr. Nasy beat me to the reply and said pretty much what I wanted to with alot fewer words. Sorry about the long post.
