Hmmm -- the "Less than 300 hours..." means little use over 3 years so engine should be close to new condition. Has it had the 250 hr service performed ?
Not knowing what the "bells and whistles" are makes it hard to say if the $115K is a reasonable/fair price. The "bells and whistles" certainly can add up.
Lack of a trailer can push on your budget. Look for a used one as new ones cost around $7K.
Have all the zincs checked... especially the bow/stern thrusters as if these are gone there could be some serious repairs required to the thruster assemblies.
Is the boat sitting in fresh or salt water ?
Is the boat docked in a covered slip ?
Any imperfections in the hull's gelcoat, and are there any bad rub marks showing ?
I cannot comment on the Cummins engine as mine is the Yanmar 4BY150.
Is it a private sale or one from a dealer ?
Don't forget any local/state Sales or Use tax that might be levied as part of the sell/buy transaction. Is the $115K excluding taxes ?
Has the boat been owned by one or more people ?
Where was the boat first purchased from ?
Where has the boat been taken, cruised to and has the boat always been in your area since new ?
Obtain the Hull number and speak with Ranger Tugs to get the boats background/fabrication/build background.
Offhand the $115K sounds a little high to me. I would not have expected this 2009 R-25 assuming a certain level of "bells and Whistles" to have cost anymore than $140K (without trailer) and excluding taxes. However, I'm no expert in the resale of Ranger Tugs, but Ranger Tugs Jeff Messmer may give you a better understanding, so contact him for advice.
BTW... There's an R-25 for sale here in the PNW area (Anacortes)... see post http://www.tugnuts.com/viewtopic.php?f=2&t=2848 and it's asking price is $115K. Compare it with your one locally.