Hi Stuart,

I'd guess quite a lot of people want to do the same as you work / carry van by day, campervan by night!  You can get seats which slide in and out.  Basically you get long rails welded to the floor of the van, running the length of the body gives you the most flexibility then you get seats on runners which slot into the rails (there are a couple of wider entry points) and these can be clicked securely into a number of points for different positions - so they can be up behind the drivers seat for sitting and slid back for opening up the space.  What would work for you is to have a bench seat / rock and roll bed combo.  The seat flattens right down and forms (all of a short bed) or part or a larger bed (often combined with another panel in the van).  You can take the whole thing clean out on a Sun night and go back to having an open van for your work stuff.

One thing would be to find a way to try and cover the rails when not in use.  They are a dirt and grit magnet which isn't a huge problem in itself but does make sliding the seat a hassle.

With the sink / stove I'd go for something standard at one side of the van (opposite the sliding door).  If you have 2 sliding doors and need this for work I'm not sure if you could sort the cooking side of things out at the back of the van?  Anyway, as you want the sink too I'd suggest you would have to do something, somewhere in a permanent unit but just design in a fairly sturdy hinged cover for it all, that way you can put it down and use that as a work surface. 

Alternatively, build a multi purpose work unit with a flat top and just go for a portable camping stove which you could put in and out of the van.   Along with a large portable water container and basin?

Ideally I'd put in some permanent storage cupboards and just rotate what is in them - plumbers kit, camping kit.  If your work requires specialist storage or is really mucky you may have to look at something flexible that can be taken in and out and swapped with ordinary storage.  Cupboards on rails??  Someones probably done it.

Not sure about the bikes.  The vito is just SWB isn't it?  Dedicated MTBs often box off the back of their vans specifically for the bikes with straps and hooks to store the bikes securely in transit.  See the MTB campervan blog and photos here.  The bed is running width ways (so not for anyone over 6ft).  You'll lose a lot of living space doing that.  It depends how much you love your bikes.  Again you could probably fashion some kind of rack for when they are in transit (and the bench seat is forward) but put them on a rack on the roof or the back door at night?  Hopefully someone else might have some good ideas .......