I've been thinking along those lines, and have a little experience. I helped one of my troops do a camper conversion of his second generation Econoline in the early '70s (no bath or cabinet work) and have looked at some used DIY conversions that included beds, seating and cabinets only (no bath, no power systems, nor RV kitchen equipment).
I can see spending anywhere from a couple thousand dollars to more than $20K for the parts, not including carpentry and cabinet work, and a couple weeks to a couple months labor (some DIY projects go on for years).
The labor is hard to figure, but you can't ignore it, as it represents the bulk of the cost of a conversion (or an up from the ground RV build). Do you charge it at what your time is worth? What a RV tech would cost? If you hire someone to show you how to do it, you'll pay for that time too. You can say "not counting labor because ... " but you really have to figure it as a cost.
The shower/toilet is one of the more difficult options, because of the systems that need be built to support it: water supply and pumps, waste tanks, water heater, usually LPG supply for that heat. For reference, a popup camper (about the only thing that now comes minimalist) adds about $800 cost to go from cold water to hot water unit, and another $3000 to add a one-piece shower/cassette toilet unit.
I've not estimated the options you've asked about, because I'm thinking of something more basic and convertible in use, like the pop-in camping systems made for MPVs and minivans in Europe.
I'm late seeing this, but my first thought on this was "go build just what I want on Sportmobile's site, and use that estimate." I'm guessing you are looking at a $15K to $25K ballpark for what you describe, which is way short of today's typical B motorhomes, which tend to put $50K to $80K of RV inside of a $25-30,000 bare van.
Pleasureway's Traverse is about as basic a camping conversion as you find today, and the fittings are somewhat more expensive than the base van, yet do not include a bath inside or out.