I built my own [1] — but then I have a garage (and had been building something approximating furniture for several years).
I feel like with the tutorials out there, a novice could learn as they go and enjoy the experience of building out a van.
It's possible too to do it in stages. My Stage 1 had no solar or stove — limited cabinetry. The wife and I took it out nonetheless. You find out quickly too that way what you miss/want.
The cost of a new Ford Transit was not cheap, and there were some pricy components. As an example, the two LiPO batteries were $1K each — but they're about 1/3 that these days (wow). The electric refrigerator (more like cooler) was not cheap. The propane stove was not either — but a camp stove would have worked as well (and has the benefit that you can cook outside on hot days when you don't want to heat up the van/RV).
I never tallied up the total cost, but I promise you it was significantly less than a new RV. And because I made it myself, I can vouch for the quality of the components.
The single best resource was faroutride.com [2]. But then there were plenty of online forums, YouTube etc. with all kinds of info.
I was a few years from retirement when I began. The idea was to "van life" with the wife after I retired. As it is, we've done a good deal of travel in the U.S. but until she also retires, we don't live the van life.
(The fact is though, after a few weeks on the road, you come home appreciating your creature comforts.)
(Recently drove from the Midwaste to San Diego to get my middle daughter and her two cats. Heading out in a few weeks to the Bay Area to visit some ex-coworkers.)
[1] https://www.fordtransitusaforum.com/threads/hot-tamale-build...
[2] https://faroutride.com/van-build/