A couple of thoughts from a guy who has had campers including a 36’ Class C:
Lose the toilet. You’re in a van, not a full size RV. Even in full size RVs, toilets suck. First, it’s a camping toilet, which means anyone older than 8 is going to feel cramped. Second, you tend to eat differently on road trips. Often this includes beans. If anyone else is camping with you, they will not appreciate your using the van’s toilet instead of the campground’s toilet, if you take my meaning. Plus, unless you get a stretch van or a Sprinter (which you don’t want because their reliability is atrocious), that toilet is going to take up a lot of room that you don’t have.
Dropping the toilet also means you don’t have to deal with the awesome fun that is emptying the blackwater tank, or keeping the enzymatic balance in the tank correct, or cleaning the stupid thing after you inevitably fail to dump enough water after every session and end up with a literal pyramid of poop in the tank just under the toilet drain.
The toilet is what turns a lot of people off of the RV lifestyle and makes them sell the rig they just bought a month ago. It’s just not something you need unless you’re planning on doing a lot of bush camping… And even then, get a camping toilet that’s designed to go outside (they come with little tents so you still have privacy).
Drop the idea of doing your own conversion unless you’re OK with spending more money than you need for a rig that will most likely have lots of problems and that you cannot take everywhere. Some campgrounds are a bit snobbish, and will not allow homebrew rigs. Also, there’s a lot of things that can go wrong with motor homes that the home builder would not think of, but that outfits that have been building them for decades have already designed against.
Also, you said you’re leaning toward a conversion van. Especially if you drop the toilet requirement, they can be had for a song these days because people don’t want to spend the gas money on them. The rear bench seat folds down into a bed, so that’s taken care of. Throw a portable Weber grill behind the rear bench seat and your kitchen’s done too. Otherwise, if you really want the motorhome stuff (fridge, stove, etc) in a van-sized chassis, look at Roadtrek. Those guys make incredible class B’s, and they’re built to last. I still see some first gen models from the mid 70’s running around from time to time. As long as you stay away from the Sprinter chassis, Roadtrek is a good option, but you have to be aware that an RV that’s only as big as a van is going to be very cramped inside. A poster above talked about spraying the stove with the shower, and that is a real possibility in those things.
If I were buying again today, I’d get a small-ish class A pusher. Good weight rating, no wasted length (class B and C have a van nose that you can’t use as living space when parked. Class A pushers do not have that problem because the engine is in the back under the bed), and because it was designed from the ground up as a motorhome, it will be more rugged and less prone to failure.