If you are dead set on a diesel, I would look at the Grasshopper with the Kubota engine.
Personally, I think it would take you a heck of a long time wear out a good gasser mowing 4.5 acres per week.
A nice thing about ZTR shopping is that there are so many good ones in the commercial class, IMO.
Take a look at the Ferris Industries ZTR. Not a really well known company but they make quality products. My parent's bought a Ferris ZTR a few years ago and with Ferris' suspension system you can run the ZTR at full throttle and the ride is smooth.
Seemed to be a really sold machine the few times I've mowed with it.
Ferris Industries Go The Extra Yard! - Commercial Mower Specialist - Independent Suspension Mowers
Get a Diesel Dixie Chopper. 7.1 acres an hour and the ability to tow a solid ton or gear. http://dixiechopper.com/media2/pdf/DIESEL_TEARSHEET_2010.pdf
The obligatory picture of their OCC chopper.
These mowers are a great way to regain the man card you lost when you bought the Prius.
Funny thing is that bike sits in the Greencastle airport, and i have heard you can paw all over it and even sit on it if you want.
I have done that, I offered $3000 cash on the spot but they wouldn't sell for some reason.........