Why pay the extra for diesel in a mower? From everything that I was able to figure out, it's not worth it, nor is there any real advantage other then the "perceived" longevity of the engine. The diesel should last 2,000 hours, gasoline 1,000 hours and a water cooled gasoline engine somewhere in the middle at 1,500 hours.
For the money, I went with a water cooled Kawasaki gas engine. It came down to just a few brands, including the Kubota gas engine, but nothing really compared to the Scag. Spindle size and the attention to detail won me over.
My Scag is 60 inches wide and almot too wide for getting a good looking cut. My land isn't smooth enough for full speed and I do have some scalping problems with that sized deck. A wider deck would be even harder on uneven lawn or changes in slope.
I paid $8,200 for mine, delivered.
Good luck,
Eddie