I don't actually own a chipper yet - trying to make moves to get a 65xp or 90xp. It seems that guys will say get the biggest bandit or morbark that you can afford, so a 15xp Bandit (or morbark equivalent) would be your comfortable minimum for those logs. You could go lower, but if the logs have irregularities you'll have problems. The 18xp is pretty popular, perhaps having to do with cdl territory, but I think someone recently mentioned that it goes over cdl if you put normal options on it. Obviously, you'll want a drum chipper for those logs.
Recent 2 year trend is away from considering only diesel - consider both gas and diesel according to your specific needs and capabilities, and get the cheaper gas if there are no obvious pluses for the diesel. The factors include your ability to work on diesel engines and proximity of the person/dealer who will work on it. I have not heard anyone say that diesel engines have improved, but they have been pretty good for a long time so I'm not sure what improvement would look like. I would look into tier 4 changes specifically - if there are such and what they are. Maybe that is the new tech that you are referring to, and if that's the case expect it to decrease emissions in a slightly complex manner rather than improve performance.
The OP discusses Vermeer, who seem to be a bit behind in chipper tech, and also proprietary (custom parts ordered only from vermeer) and big box in nature. Be aware if you go that route. Here in Florida, everyone has Vermeer, which makes the used market kind of annoying for me to navigate. I also have to figure out who will repair a non-vermeer chipper if I go that route. Wish you success.