here's my 10cents after ~60k miles with our interior build
1. Overall, just have a well devised plan before you begin. Take the extra time to learn how to use Sketchup or something similar so you can start to model what you are going to put inside. Then if you have the time, use some boxes or totes to mock up physically placing things inside the van. This was critical in my build for determining the height of things inside the van (especially seating) so that I knew I would always have enough head room to sit up straight. I easily spent 100+ hours studying other builds, sketching, measuring, noting and learning what I DID NOT like from other builds (and figuring out how to avoid doing the same thing) before buying any supplies.
2. If you plan to do any sort of insulation (I def recommend it if you're doing a camper), buy trim tools and use them appropriately to remove any interior panels. Take the time and effort to do this correctly and methodically so you dont break anything or lose any hardware. It will give you the chance to clean those very hard to reach spots, repair anything if needed (and replace your rear speakers?), inspect interior panels for damage/rust, find old japanese coins (or other things stashed in the walls), install sound deadening to the big open metal areas, and if you plan to run any additional electrical for your build - this will be the easiest time to do it. You can run things behind the panels and keep the inside looking nice once its all back together. Ive learned that my lack of planning early on for some electrical additions has made some things much harder. Also, the headboards are the hardest to remove IMO cause you have to remove a lot of other pieces first, but the extra time doing this was totally worth it to me in order to add sound deadening to the roof and add a layer of thinsulate insulation AND some reflectix, and hide it all behind the stock headboard.
3. In hindsight I wish I had used Havelock wool instead of thinsulate. Thinsulate isnt bad... its just that Havelock wool is just a better insulator overall. I regret not using it because it'll be a real tough project now to replace it. If you do go the route of using Thinsulate, the 60' x 5' roll is enough to have plenty of extra for mistakes.
4. I am really happy that I got a hydronic heater and tapped the rear blower system to our Aux battery for heat. This has taken up no additional interior space, provides great heat in the winter, and serves as an engine block heater. While expensive and a PITA to install, i am soooo happy that I spent the money on this. Winter camping is enjoyable and the inside stays warm all night. The rear fan/blower does not draw a ton of current, and can also be used without the heater as a supplemental fan to circulate air in the summer while camping.
5. As others have said, having a good spot to cook is critical. We cook out of the back with the rear gate open. This is awesome in the spring/summer/fall. IMO it sucks in the winter and even worse in cold wind. Any heat you had inside the van is gone quick. If you cook out the back and can come up with a way to keep heat in while having the back open (maybe some sort of curtain), this would be nice...
6. Ceramic tint on the windows and curtains are a must for summer camping to help keep things "cool". Get a small 12v fan with a clamp and hang it from the handles (or just use the 12v power tapped rear blower with the rear vents
)
7. Figure out a way to have a curtain behind the front seats. We hung a thin piece of aluminum rail from the vents, and made some insulated curtains from ripstop material. Black facing out, grey facing in (to match the interior). This is great for privacy in general, and also helps keep things a little more stealth (nothing says "someone is in here" like reflectix covering all the windows). Curtain also makes the space easier to heat in the winter by keeping the space smaller. Adding this curtain was a big improvement to our camping (and boondocking) experiences and wish I had done it much sooner.
8. 1.5 years after the build (and 20k miles later), I added a 15A shore power hookup. I'm using a 15' Noco cord that goes from my rear bumper up through the air hatch. From there I have a Victron 15A battery charger plugged in going to our 12v aux battery. Having shore power and this battery charger is amazing and changed everything... but its kind of big (like the size of a big shoe). I don't have anywhere to put this battery charger, so its just on the floor in the way. I wish i had a spot for it. This summer I may try to figure out a place to put it inside the wall of the van behind the panels.
9. not obvious, but something I've come to love - get a small rubber floor mat that you can put right OUTSIDE the van when you are camping. gives you somewhere to take off your shoes before getting in and in my experience has helped keep the inside cleaner. We just roll it up and strap it down with a bungee in our roof basket when not in use. This was an add-on we picked up while on the road.
10. Last but not least, if you have the ability and space for it, save all the parts you remove from the van if they are in atleast decent condition. You never know if you'll wanna put your old seats back in. Almost 3 years now with our camper build, and I reminisce the days of just chilling with the stock seats in place. They are taking up space to store, but I am glad I still have this option if I ever choose to put it back.