Interesting options! The 525hp Cummins is the N14 and it is quite the monster. Big motor for the coach and could be a bit of a hotrod. The N14 was only offered for a limited time and I would think these coaches are quite desired. The 450hp Cummins is the M11, which became the ISM and was the standard engine option going forward for many years. Also a great motor.
Starting in 2000, Monaco built all the Signatures on the tax axle chassis including the 40 foot models. Weights were getting too close to the limit, especially as more slides were added. Not sure if your carrying capacity might be more limited with the 40 foot single axle. I personally would be a bit hesitant. Of course an extra axle means extra tires to inspect and check DOT codes on.
65,000 miles on a coach that is 21 years old is on the low side. Sometimes low mileage can be worse than high mileage, if the coach has sat for awhile. These coaches are meant to be used, when they sit for awhile it’s never good. This will come down to the history of the coach and any service records you can find. I wouldn’t just take the owners word for it, do your best to inspect it and seek out maintenance records and other helpful information.
Personally, we are big fans of a tag axle coach and wouldn’t buy another one without it. Anyway, just some thoughts based on the info you have provided.
Curious to see how you make out. All the best!