My Ferris 4500IS has the 28hp CAT and plenty of excess power for a 60" deck cutting 8" tall fescue down to 4.5". I think the ~33 hp CAT on the 5100IS would be more than adequate even if you went with the 72" deck; I can't see the need for a 4 cylinder. I believe the Ferris "CAT" is actually a Perkins diesel manufactured and serviced by CAT; I'm sure it will outlast me.
The machine is awesome. The
only drawback I've found is the heavy weight results in compressed grass wheel tracks --- looking in your posted picture in the background I can see where you drove the machine. Given the country setting this is a non-issue, but if you were a commercial landscape mower some businesses might find the tracks objectionable. Maybe I"m cutting the grass to long.
Ferris has been making mowers for 20 years and has been bought by Simplicity then Briggs and Stratton, so they "probably" will have staying power and not file chapter 11.
History Bad Boy has been around for <10 years, and didn't have any inventory for me to test.
Who We Are | Bad Boy Mowers
Simplicity has the patent rights to the Ferris IS so if you wanted a cheaper "homeowner" version you might check them out. But, from the looks of your land the Ferris diesel is the way to go.