The F550 is very popular for the smallest drills, even the CME 45 and Mobile B42. From what I've read, there were some issues with Ford diesels, especially the 6.0L that they used from 2003 to 2007. Even the newest diesels can be snake bit. I like the gas V10 even if it is a little gutless and gets less fuel economy. It's cheaper to buy, cheaper/easier to maintain and proven. They use the V10 in class-A RV's, so it can handle an F550 with a small drill mounted on it. You didn't say what kind of issues you had, so this may or may not be applicable to your situation. An F650 would have absolutely no problem at all with a D25 and probably be $$ overkill. You can't get 4x4 in a new F650, though.
On a side note, I have been trying to find information for the D25 as I have never seen one in person and nobody has one (that I know of) in the entire state. A CME 45 is sort of the entry level rig that is deemed suitable by drilling subcontractors for the majority of basic jobs around here with up to 35 ft dry auger depths in stiff to hard clays and soft to moderate rock. I understand the D25 has a smaller engine, but a larger range transmission. Can you say how this drill compares to the CME 45? How does it handle stiff clay and soft to moderate rock (limestone/sandstone) using dry augers? What is the quality, durability and reliability of the D25 like? I should probably stick to a CME 45, but considering the nearly $45K price difference for a new drill, the D25 looks very attractive, especially since it may only see two to three small jobs a week, with tougher jobs being subbed out.