Contrary to what advertising and social media trends would have you believe, most people who choose vanlife don't decide to go there because they've started with dual tech industry incomes and the ability to deck out a $50,000 Sprinter with all the amenities of a highrise condo.
It is absolutely possible and sometimes necessary to make reasonably small adjustments to an existing van to make it livable on not only a budget, but in a tight timeframe, in which you just can't spend 3 months with your new house, that you've gotta actively be living in, sprawled out in pieces in some garage somewhere.
For some, even affording a cargo van vs. a passenger van, which are generally not only cheaper but come with built-in environmental controls for the back living area and have some (but not much) insulation, can be out of reach - as was the case for me when I moved into my van in 2015, and needed what little I had left over after buying the rig for ongoing maintenance and inevitable repairs.
Here's a little image montage of my friend Chris and I making the sleeping and storage platform I conceptualized for my 2002 Chevy Express, currently humming along at 210k miles.
I prioritized figuring out a way to make the seat brackets in the floor work for rather than against me, using the thick camp pad (which folds into a chair) that I had already been sleeping on, and keeping as much open space as possible in the mid part of the van to be able to set up my piano keyboard (I am, among many other things, a musician).
This is still my basic setup 3 years later. The platform opens from the top to allow access to the stuff underneath, and at 5'8", I sometimes have to get a bit creative to stretch out enough while sleeping without burning or freezing my toes or head on the glass, but I sleep pretty darn well in here overall.
I also got a short plastic kids table and wedged it between my front seats to create a kitchen area where I could use my camp stove to cook as well as creating another semi-permanent storage option, and got rid of that one bench seat I still had for the car. Adding some LED string lights shoved into the seams of the plastic finishings inside the rig, and you've got yourself a cozy little bedroom too, which is a hell of a start to a home.
Overall, this approach has worked well for me, though I do fantasize about geeking out to build something more substantial, and I do watch youtube vids of what other people are doing with their van builds. For me, I would like to at least stick with the super basics while I am committed to making this particular van work. When and if I can ever upgrade, my first priority after 3 years of this rig is getting a vehicle tall enough that I can actually stand up in. Everything after that is pretty much gravy.