We transitioned from a CDory 22 to R25 inboard 12 years ago. As mentioned above it compares to switching from tent camping to a motor home. The comfort of the R25 makes it much easier/more pleasant to spend more time on the water. But I miss the simplicity of the CDory and the ability to explore and anchor in tighter spots. Dinging a prop on the CDory just meant installing the spare on the water. Ding a wheel on the inboard and your poking home with a kicker or calling SeaTow.
Regarding using engine coolant for cabin heat and water heater, water yes, cabin heat not so much. Maybe farther south but on Prince William Sound a diesel heater is a must for cabin heat. Plus keep in mind it only heats while the engine is running. Water heats up in an hour or so of cruising and stays hot enough to shower for several hours thereafter. Cabin heat stops as soon as the engine shuts off.
When it comes to cruising speed if you plan on extended cruising you can forget about the advertised speed. With full fuel/water tanks, 10 gal for heat and spare fuel, 15 gal extra water, two adults, a dog, and typical cruising gear we were doing good to make 11-12 knots at 3200rpm. But the real issue is range. You're looking at one gallon/hr at 6 kts vs 6 gallons/hr at 12kts. So 6 nmpg or 2 nmpg.
At the end of the day we decided the benefits outweigh the added complexity etc. It's a comfortable, reasonably seaworthy little boat as long as you're not in a hurry to go anywhere. After much consideration of alternatives we decided this is our boat for the duration. With 2700 hrs on the original Mercruiser/Cummins we just re-powered with a 110hp Yanmar. Just about to roll over the 50 hr break-in period. With your 2500 Duramax you won't even feel it back there. So the miles that you can't cover on the water you can make up for on the road.
Hope you find what you're looking for and enjoy it in the upcoming years.