Fair enough ofc, it'll work but i reckon is gonna be real overkill for a little bongo. If I was doing it in a rush for someone else Id probably use 9 or 12mm ply with corner blocks, especially in a small van like a bongo.Your conversion sounds fantastic and what I would probably aim for for myself. In this case it’s for my daughter and I have be allocated the entire bank holiday weekend to start and finish it.
It’s down to availability, choice of finish, available accessories (like corner channels, t edge, etc), ability to pocket hole and not needing an internal frame. One and a half boards is not going to be an issue for weight as I have removed the old unit which was built from 18mm ply. Having also removed the seats I’m going to be lighter overall.
Make the cabinets, scribe to fit, bolt into double skinned metal (and floor too if you can) with some decent L brackets & rivnuts.
Put a decent worktop on it and itll still look nice.