Got the latest transit custom crew cab with the rear kitted out for bike storage including an inverter to charge the bike, dry boots and power a small washer. Works like a dream. Don't use it for sleeping but it drives so well it's now become my main transport.
Think you'd need to start from a Tourneo rather than a transit as they have lots of individual seats in the back, you'd just need to black out the rear windows.
It's a fixed 3 seat bench bolted down. Probably could be removed but definitely not designed to be removed regularly - I would guess it weighs a tonne?
Standard length transit double cab I'd say you'd get 4 diagonally.
Front wheels off you can get 3 lengthways but I doubt you'd get anything longer than a medium. Both my bikes are smalls and I'd guess there's only a couple of inches spare.
You can fit a few bikes diagonally with wheels on, but I take the front wheels off and fit them straight on using a bikeinside rack. I'll post some shots tomorrow.
I put a 1000w pure sine wave inverter in my transit and it works a treat. The van has 2 batteries anyway as its got stop/start. So far 3 months in and its never missed a beat although I only charge the Levo whilst I'm driving.