ELECTRICAL
The price of a Delta 2 + its auxiliary battery is about the same as the Delta 2 Max, which has the same capacity in a smaller footprint and a more powerful inverter. It sounds like the electrical system you're planning is on the larger side and will cost a significant amount of money, so here's some things to consider.
When in comes to power station vs permanently installed electrical system, I am firmly in the power station camp. But there were a couple of shortcomings/pain points that developed as my electrical needs grew.
First is the limited number of ports. For example diesel heaters and fridges both require 10A, and there's typically only one port (cigarette) with that capacity. Its also an issue if you want an appliances permanently connected, like a fan or lights. It doesn't take a super complex build to run out of ports. One solution is to run wiring then route it to a distribution box connected to a power station that has a 25A or 30A port. The only model in the EcoFlow range that has this is the Delta Pro. All of the large power stations in the Bluetti lineup have a high output port. Ditto with Goal Zero, but it has inferior battery chemistry. These power stations also have a 30A 120V port if hard wired AC appliances or outlets are desired.
The other issue is recharge rate. If a power station is small and there's nothing in the build that demands electricity, its a simple matter to grab it and take it in to a building to charge it. But once you start talking about larger power stations or additional batteries or always on devices (like internet equipment) removing the power station from the vehicle can become a nuisance. With a limit of 8A, a vehicle's cigarette port isn't practical to recharge a large power station. There are ways to connect a power station to the starter battery so that it can charge via the alternator. From (costly) experience, I can tell you the safest (though not fastest) way to do this with the EcoFlows is use a DC-DC charger. One more thing, the Delta Pro can be charged at EV charging stations. I've found this to be a surprisingly convenient way to recharge on several occasions: drive into a spot, open the sliding door, drag the charging cable into the van, and 30 minutes later, my deeply discharged Delta Pro is full.
HEAT
In the Mid Atlantic region, you will need a heater if you are going to be living in the van during the winter. If you have access to shore power, an electric space heater is simple, cheap, safe, effective, and silent. I needed something that worked without hookups. I started with a Mr Heater Buddy heater. It gets dangerously hot, consumes a lot of propane and creates unwanted moisture. I don't want to start an argument about safety, so I'll just say it made me uncomfortable.
In my opinion, the best DIY friendly solution is a diesel/petrol air heater. I have zero mechanical skill so I bought a Chinese all in one diesel heater, plugged it into the cigarette port and routed the exhaust pipe out the window. The setup and take down was a hassle and the slightly opened window let in a lot of cold air. In hindsight, a better option would have been to drill a hole in the van floor and route the exhaust out it. I could then remove the unit to put fuel in it outside of the van, and would have the option of leaving it at home during the months when heating wasn't needed. I ended up getting someone to install it traditionally (under the passenger seat and plumbed to the vehicle's tank). If you have a diesel Promaster, a cheap Chinese diesel heater is a no brainer. Espar is the only company that I'm aware of that makes a petrol heater. Plumbing these heaters to the vehicle's fuel tank makes it safe, clean, odor free and convenient.
If money and complexity are not a deterrent, hydronic heating seems really neat. I am looking into the Elwell Timberline system for my next build. Having hot air and continuous hot water powered by the vehicle's fuel tank may be the most elegant solution.
PLUMBING
In many situations (and most urban/suburban areas) it is poor etiquette to dump grey water (or urine) on the ground. There are numerous times throughout the day when its nice to have running water to wash hands or rinse off a piece of fruit or clean the dishes in a convenient, easy way. In a full size van, its hard to make a compelling case against having a fully functioning kitchenette. It takes very little skill, money, or time to create.
Home Depot and Lowes sell inexpensive, fully assembled kitchen cabinets (and bathroom vanities that can be repurposed as such) with countertops. My initial kitchenette consisted of of this ($200) and two Aquatainers ($40): one with a hose connected ($3) to a battery powered water dispenser faucet ($20), the other connected to a rv drain hose ($20) and a pvc connector ($5). Total cost $250 and a couple of hours.
Living in a vehicle is hard. Especially if constant mobility is a required. Finding places to park, shopping for food, cooking meals, doing laundry, bathing, toileting, consciously monitoring water and power consumption... Everything takes more time and thought to execute.
A van has roughly the same square footage as a standard bathroom. Now imagine living in that space without hot running water, fridge, shower, HVAC, 55" tv, recliner, unlimited high speed internet... Make sure your build has the elements that make living in a vehicle enjoyable and doesn't make you feel like you're homeless.