Sticker glue remover: acetone
Window blackouts: Custom cut padded silver sunshades (curtains hang straight, not as good)
Mattress: Try an Ikea sofa bed one - they're pretty comfy and fold easy, reasonable price
Padding / soundproofing / flooring / insulation for wall and ceiling cover: 1/2" Thick foam floor matting - cheap, interlocking tiles, can cut to fit
Space arrangement: I'd put the bed right up the front. If you've got a lift up back, that will give you some shade / rain cover and you can set up your kitchen, seat, etc on the tailgate. If you're getting fancy, try the boat fitters for ideas - I saw a van in Germany once, it was a-may-zing. All mod cons in a kombi!!! Fold down shower well, etc, etc!
Secret / locked spot: Don't forget to build this in for your valuables
Make sure you post some before / after photos!!