** Trying to stop laughing at BetonBlack's response enough to type a decent response. Ok, here goes:
This is a great question. I'm sure it's been asked before but I like your approach. I've built several and I have no special skills. 5 or 6 years ago I realized how much vans call to me and how universally utilitarian they are and I've pretty much adopted them as my only vehicle needed in life. Well, maybe a sports car in the garage. I certainly can't follow through with selling my old BMW.
OK, moving on. I like your list but here's how I would ammend it. 1, 2 and 3 are things I would do really before I bought it. I'm always imagining different floor plans and have 10 or so perpetually improving ones in my head. Making sure the vehicle side of the van is up to snuff is kind of a given. For me, after taking ownership of the van I would throw what you need in it for family, work, commuting and light camping and just start driving it. How you move about in it and how your floor plan will come together, for me, comes from spending time in it. I'm sitting in my Transit right now (camping while on the road for work) and staring at the rear half trying to figure out how I want to do the cabinets. I've been doing this for a month. So, that's #1 for my list. Use it a bit and gauge your needs.
#2 Lay out your basic electric and plumbing. If you go with modern technology (vs. old school RV guts) there will be very few holes in the van. More importantly than cutting those holes at this stage is checking to see if you can
cut them where you basically want to place things when the time comes. So, recapping, plan and lay out your plumbing and electric. Drag extra 12-14 gauge wire EVERYWHERE! It's cheap. You will inevitably want a light or switch here or there later and it's easier to do this now. I've only started to do this on my 5th or so van build. Run extras behind the walls and under the floor from side to side. If you plan on using Romex for AC outlets (via shore or inverter power) then run extra Romex. It doesn't hurt for wiring to lay there unused, and it's a pain in the ass to run it later. I recommend running at least one run (or circuit) to the driver's side cabinets, one run to the passenger side cabinets and one to an outside outlet, preferably for most campers by the side door somewhere, maybe even hidden under the van in a waterproof box. Don't forget your shore power inlet run.
#3 Insulate and do your floors and wall and ceiling panels.
#4 Start building cabinets. Don't fret if you build them more than once. I usually do. I think I've built almost every good cabinet that I really liked at least twice. Vans are getting more square so this is getting easier to accomplish. Ford Econolines generally take 15 or 20 builds due to not having one single straight line in the design.
#5 IMO all exterior mods, even a 4wd conversion can really just happen when you get to them, at any stage of the process. Inside and outside mods are generally exlusive, except for water/waste tank and battery mounting if needed on the exterior. I'd recommend doing an exterior battery mount early on with any van since the house batteries are more or less the foundation of your electrical system.
*The most frustraging part of a van build to me is that I prefer, and benefit from, using the van while I'm building it, but this is hard. It's a lot harder if you don't have a dry, warm place to work on it and store your tools and supplies, but it's hard to me to stop and start. I'm doing that now working out of my van during the week and trying to get my head fully into building on the weekends when home. It's a slow, painful process.