You may want to post on the tractor forum. As far as the engine comparison, search with the specific engine #s for issues. Kohler has noted changes in their relatively newer Courage line pretty consistently. All in all the twins have performed well. $200 is chump change compared to what it will cost to replace an engine on any ride on.

IMO Kolher courage is no Kohler Command Engine which Kohler has built good name. IMO they are low budget engines to complete with Low budget Briggs Inlek. Nothing wrong with that as long as you service it regularly (regularly IMO wouldn't be the 50 hours schedule but the schedule for the conditions you operate in. in extreme conditions it could be 20 or 25 hours or less (read the manual under operating in extreme conditions))

As for which one I would go test drive them see which one you like with the features you want. Dealer support and convienence could be desiding factor if he's rated highly .

Is the engineered design life of the Kawasaki 726 cc FR equal or better than the Kohler Courage PRO?

Defiantly a Toro, toro are 75% of the worlds leading market in zero turns they are quality above the rest. i would be looking at a TimeCutter ZS5000 probably

Here is a link that might be useful: Toro TimeCutter