Hi Steve,
I cannot help with the dimensions but I built my own motorhome and two mates have done the same. I would suggest that you go and see as many built van conversions as possible first and don't tie your self to having a Sprinter or Crafter.
It depends upon age but they tend to rot if not looked after especially high mileage. Look what converters use and at the moment they are going for Ducato and Citroen and there is good reason. They are very square as many vans taper at the top which you lose space. If I was building another that is what I would chose, spares and servicing available everywhere in the UK and EU.
Mine was built 17 years ago and still looks good but fading paint as commercial vehicles tend to have a quick splash. All the bits we bought from Magnum Motorhomes in Cleethorpes trim wise and we built everything else. If you buy proper caravan motorhome products the weight is very light, it is easy to use normal thickness goods and have a heavy van, leaving little for towing.
Upholstery material off Ebay. It has shower and toilet but I did not go for on board water and waste as it is unnecessary weight to drag around.
The van was two years old when bought and the first thing I did was go to town on waxoyle everything in every box section as you cannot get to them afterwards. Also make sure that the trim fasteners are siliconed, the sprinters are bad for water ingress from them.
Good insulation is a must. Plan you electric runs early on.
Pictures below what you can produce with a good plan.
HTH