Going by your profile pic, are you going to want to carry bikes in the van?
I'm mid way through designing & building the back of mine, it's a Caravelle so everything attaches to the factory seat rails and is fully removable to return to standard.
I've removed the bench seat to be left with 2x single captains chairs in the rear, which gives me a 4 seater for normal every day use.
I've then built a bike drawer which can take 2x mountain bikes, one of which can be basically the longest MTB available. And the 2x seats in the rear are still fully usable with the bikes in.
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The next stage (which I should have completed in the next 2 weeks) is a kitchen pod behind the drivers seat, with a sink and tap, 50l fridge, 2 burner (portable) gas stove, and 16l bottles for fresh and grey water. Oh and a 40mm solid walnut worktop, and pull out table.
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Then, the third stage will be to build an extendable, foldable frame that will go over the 2x on seats when folded flat, and be stored behind them when they're in use, which will be a 6ft long, single bed.
That's the main bits - my brief was that I needed 4x seats, to be sacked to carry 2x bikes, a full kitchen, and be able to sleep inside the van (just me) along with the bikes. Literally no off the shelf solution caters for all this inside a SWB T6!! No pop top, as it's a caravelle they're a pain to fit. Maybe later. That would allow 2x people to sleep in the van.
I'll be fitting window bags to the rear windows, along with curtains to the rear, side and a cab divider curtain. And later I may build some more storage on the right side, with water storage for bike washing.
The van came with a leisure battery, eberspacher heater, awning, and swivel passenger seat when I bought it.