Seriously, build your own.
Choose a vehicle YOU like, that's going to suit you / the application. You're the one going to be living in it !
Somebody elses cast offs are always going to have THEIR tweaks, you may not like, and you'll be forever faffing about changing their preferences to yours. Somebody elses van has ALWAYS got old cables going someplace, that lead nowhere.
Build your own and you'll know where everything goes, what the limitations are, and it'll be a ruddy site easier to adapt as you go along.
For f's sake, keep notes / diagrams. Trying to work out what you did, when, why, keeps blokes like me in business.
Boarding, insulation, carpeting are cheap, easy DIY. Sorting electrics best done by local auto electrician. Just tell him where you want the volts, how many, and current consumption.