There are a few things that are required for it to be registered as a motor caravan. So they are on the "must have" list. Some converters struggle with the requirement of the length of bed, which is silly. The length can be measured corner to corner, like a TV screen, because 'length' is not defined on the rules.
The top priority I would add is a combination feature:
You need good insulation, but it MUST have an effective vapour barrier on the warm side of the insulation, or the bodywork will rot away.
You also need to make sure that cushions, bedding etc has ventilation on their cool side, so slatted (not board) bed bases, small panels to keep cushions off the walls.
After that, it is more a matter of choice. I would design in a toilet, because they make the van vastly more useful and hugely increase the number of places you can stop. But not everyone agrees.
I would install a gas tank, or at least a big refillable. Use gas wherever possible. It's cheaper and lighter than electrical power, Even "free" solar power costs more than autogas.