A 5-8 year old coach will be 15-18 years old by the time you plan to get rid of it. At the high end of your expected usage, you will add another 30,000 miles to the existing mileage on whatever coach you buy.
You will not wear out the engine, whether it is a gas or diesel power plant. Your friend racks up a lot of miles, so for him a diesel MIGHT be the best choice. Many of us have had both diesel and gas coaches. I have never heard of anyone on this forum actually wearing an engine out, whether it is gas or diesel. There have been instances of both engines having catastrophic(very expensive) failures but it was due to some other outside cause rather than the engine being worn out from just racking up the miles.
Based on your description of the type of trips you plan to take, I would recommend a gas coach and save the extra cash to buy other items. With short trips the larger fuel tank is not very important. With your schedule, you will be spending more time parked at your destination than driving down the highway.
I would concentrate more on a floor plan that you both like and put the type of engine way way down on my list of things that have any importance in your enjoyment of the RV lifstyle. .