Here's the Ferris Specs
. Looks like the IS 500Z uses independent pumps and wheel motors. If so, that puts it into the professional category, and accounts for most of the price delta. The other machines you mention would be more prosumer (top of the consumer family) as they use heavy-duty IZT drives. So yeah, I have to say the Ferris is an orange.
You'll find more manufacturers trending toward HD IZTs (ZT3400 and ZT5400) even on what was traditionally considered "entry to mid level commercial" models, so there's nothing inherently wrong with that drive system. I'm thinking eventually, all manufacturers will change over to HD IZTs except for maybe their top-of-the-line, $10K+ machines.
For a homeowner with a large property such as yourself, I think one of the best bangs for your buck right now is the Husqvarna MZ6128T