kaz, i got an ex bt van with a box on the back.it had a heater in it and i joined the cab and box to make it walk thro'.
in the back i got a cooker / sink and fridge out of my old van. fitted a small wardrobe with some shelves.
you can buy windows to fit and the walls of the box were twin skinned with foam between them. ( there were square section tubes running verticaly so the walls were nice and strong.) i used foam backed carpet glued on to the walls to make them nice.
i would say any seat/bed arrangement you go for make sure it's easy to set up and is nice and sturdy.
make sure you have loads of cubby holes and storage places, you will find they come in really handy.
the type of water barrel that rolls along with a detachable handle is one of the easiest options ( just have it on the floor outside the van and use a small 12v pump on the sink and a hand or foot pump to prime it. )
make sure you fit an auxillery battery for all the electrics you fit in the back, don't fit anything to the vehicle side of things. 'cos you will flatten the battery by accident one day. if the aux battery is a good quality leisure one it could also be used to jump start the vehicle if required providing the jump leads you carry will reach between them! oops! guess who's first set didn't?
there are loads of things you can do and there are books out there that tell you about conversions.
one word of warning, if you employ a company to do the conversion the cost will probably be higher then the value of the van after. my mate got caught out this way, �4k van + �3.5k fit out. value after ..... �4.5 to �5k
lastly.... make sure you can stand up straight in the back !!!!!
what ever you do enjoy! mark.