What's your budget? That will certainly narrow your focus and provide more meaningful responses.
One thing for sure, if you build your own, you will know every nut and bolt. My wife wanted me to build a skoolie and I talked her out of it. While pretty cool, they ride awful and have almost no storage. Let alone that everything needs done. Meaning, unlike an MCI or prevost with multiple AC's, outlets, and a bathroom already, in a skoolie you will need to handle all of the electrical, HVAC and plumbing. The other thing to consider is that while one-off's and totally unique are cool, you'll never get your money back out of it. It will be a labor of love. RV's are never an investment but most folks buying RV's lean to production models with a greater support. It will be a labor of love. And nothing wrong with that provided you have realistic expectations.