If you go with a scratch built interior I have an idea that I have been playing with. Plastruct has a very thin sheet of plastic that looks like Tuck & Roll upholstery. The part # 91519 is O gauge corrugated siding but it looks perfect for interiors. It is corrugated on both sides which make it very easy to bend if need be. The interior in this kit isn't very accurate as I'm sure you have found out from your research. I really like your decal idea, is this something you have thought about selling?