Steve..
I am not against propane, I use it for heating..I have 1000 gallon tank. I have had Gasoline, Diesel and Propane tractors before,
Propane is a reliable fuel, it is just not handy. You can easily fill gas, almost as easily diesel... but propane uses more gallons and is not so easy to refill. It is cleaner but the refill cost and bother is my only problem with mobile use.
I would definitely need to drive a zforce around if I were you, I would also try out a few ZTR's, once you have tried one you will be hard to sell anything less.
If you are wanting to last over 10 years you really should be looking at a commercial grade machine. You are mowing 2 acres, so it is quite a bit more than the average homeowners lot size.
A multi blade deck needs plenty of steel to hold the blades level while under load. You also need enough metal in the frame to put up with a lot of twisting and turning. Check blade spindles they should have grease zerks, wheels and suspension or steering points should also be serviceable, too much is sold today with 'sealed' bearings or light wt bushings, or even plastic inserts as a bushing... these things can not last 10 years...
The zforce dose not look terrible BUT for the price I think it is about $2000 too much, some of that is the increase for LP fuel but still for $6249 I would expect a bit more than that has to offer.
The same Kawasaki engine is in the gasoline and LP model...