The camper van will have basic acoustic and thermal insulation on walls, floor and ceiling.
I built a suspended kids bed for our one year old son. It fits the Mitsubishi L300 4WD, but our other campers as well.
I bought two rear seats out of a Renault Espace including mounting rails. Safety-wise they offer integrated 3-point seat belts and isofix to mount a childs seat.
I’m installing a kitchen under the bed. A Waeco/Dometic Coolmatic CD30 drawer fridge is perfect for this.