A 25 hp riding mower with a 48" deck is just sign that you have no idea what you need and you like to buy gasoline. The old JD 316s/318s are great mowers with plenty of power and "just" 18 hp. Riding mowers have more and more power every year because the hp sticker sells them to clueless buyers who think the extra power makes them the envy of their neighbors. Nevermind that if you mounted a torque cell inline with the engine they would likely never use more than 20 hp over the life of the mower.
That said, Dargo is correct in that hp is hp and torque is torque regardless of what fuel source it comes from. The things you've mentioned Dmace are accounted for in the measurement of the output of the engines. You are more likely to be disappointed in the power of a gas engine at or near idle vs. a diesel engine at or near idle but if both engines are operated at the rated speeds power should be identical.
My CK20S (just 22 hp) is tremendously powerful in low range. I've yet to stall it pulling my middle buster plow, I run out of traction first. I wouldn't complain about a few more horsepower for use with some PTO attachments but the tractor is very capable overall.
When you're using your FEL or backhoe the hydraulic pump is only capable of using a portion of engine horsepower anyway. This is even more relevant on the JD/Kubotas of similar size. If you buy your tractor without regard for its capabilities other than horsepower are you any wiser than the guy buying the 25hp riding mower from Lowes?
Sounds like an ok price for the '07 CK20 with FEL, backhoe, brush cutter, and back blade. $13,500 would be a good price. To purchase the same equipment new would run somewhere around $18,000.