Conversion
Rework in a van build is expensive because everything is fastened to everything else. Do the invisible work first.
Decide the layout before anything else
Bed position, where water lives, and where the heater exhausts decide where holes go in the body. Mark them on the floor with tape and live with it for a weekend before committing.
Insulation and vapour
Closed-cell insulation that does not absorb moisture is the safer choice against bare metal. Cover as much exposed metal as you can — every uninsulated rib becomes a cold spot that drips.
Electrical
Size the battery bank to your worst day, not your average one, and fuse every circuit close to the battery. If you are not confident wiring DC systems, this is the part to have checked by someone who is.
Furniture last
Build furniture after the walls are closed and the wiring is tested. Leave access panels to anything that might ever need attention.