Any info on the "12V compressor & condenser" you are looking at using?
Also, what advantages do you see in hydronic heating over diesel air heater and electric water heater? I have been leaning toward the latter for simplicity and longevity. The diesel heaters seem to have a limited lifetime but are cheap to replace.
Have you considered the resale value of the van if you go with a custom "non serviceable" HVAC solution?
The main reason for not relying on electric for hot water is the power draw when off grid. With hydronic heating, most of the energy comes from the gasoline which is already on-board and more cost effective than battery storage as a source. With electric heating, raising 7 gallons of water from 70 to 120 F would pretty much draw down a 12V 100 AH Lithium battery to 28% and it would take 36 minutes with (my 1440 watt) electric heater to heat it . This all assumes perfect efficiencies which there never are. I do plan using an electric heating element if we are connected to shore power.
The second reason is that I can use the AC blower and ducting to more evenly distribute the HVAC airflow. I think it's going to be quieter, but I'm not sure. I'm looking for some noise info, but haven't tallied it yet.
The third is to consider using the heater to freeze-proof the fresh water tank via a little recirculation, not something I would want to do with batteries. In cold climates this could seriously impinge on remaining battery capacity.
I don't consider the A/C system non-serviceable because there are nationwide service centers for it. The heating system's core is from Eberspacher, which I don't consider non-serviceable. The jury is out on the surrounding other heating components, potentially from Rixen's. It looks like they make quality stuff though, just not sure about servicing yet. I haven't talked to them.
Nevertheless, I don't put resale value high on my priority list. I've been camping all my life in multiple setups and plan on keeping this conversion until it is almost fully depreciated / worn out. I do agree though, that this should be considered in case something unexpected happens, just not giving a lot of weight right now. I would rather get what I want to use than sacrifice for what might happen in the future.