JD's website says this is gas, and it's shaft drive. Looks like a nice tractor. List price is $9699 (new prices as of Nov 1).
You need to to a 74x (740, 748, etc) to get diesel.
I have the 54" EDGE deck on my X500. I mow about an acre with it. It's fast and the deck will cut evenly at all but the very highest speeds. On left turns, it tends to leave an uncut strip unless I go slowly. Also, it's not going to cut as well as a front-mounted deck, since the front tires on the X-series tractors will always press the grass down so that even mower lift won't pull it all back up to be cut. But that's the price we pay for having a tractor-style riding mower -- you get to use it for things other than just cutting the grass, and the X700 will handle a lot.
I have a snowplow and a rear box scraper on my X500. I used the box scraper a couple times to spread gravel. Try doing that with a zero-turn!