You’ve been listening to the wrong people, no honest person would suggest that a newbie buy a van and build it out. The best bang for your buck is an older factory camper van or factory built motorhome. Both cheaper than building your own, and better built as well.
I always suggest that newbies rent a small motorhome, and a camper van first. Try them each out for a week or more, to get a decent prospective of which you like better. I have bought many for under $2500, put over 100k trouble free miles on them, and then sold them for the same or more than what I paid for them. The secret is to buy one that has been fully depreciated but still in good shape. The older ones were built better too, both the vehicles and the camper portion. Newer isn’t always better.
I would never recommend a camper van for more than a single person, not to live in. I’ve got one of the biggest vans available, but with two people it becomes too cramped almost instantly. Sooner or later we all get trapped in our rigs for days, and maybe weeks due to the weather or other circumstances beyond our control. You really want something that you can both be comfortable in when that happens. For this same reason you want something full featured and self contained with a toilet, shower, and full kitchen.
Diesels usually cost 3-4+ times more to own and maintain than a gas vehicle in the USA. Other countries may vary. The less we spend on a rig and it’s maintenance, the more money we have to actually enjoy our adventures.
"Happy Holidays!" ~Van_Dweller