Hello Maryrrf,
From my experience of rebuilding a coachbuilt 'van some years ago I would not recommend anyone amateur to attempt to convert an existing van (Transit type). The costs are so high that you would never recoup your expenditure. The resale value of a home conversion is very low.If you ever wanted to part-exchange for another 'van a dealer would not touch it!
Just go to a large store selling caravan equipment and look at the prices for the basics! A fridge is nearly £500 and a cooker over £200. Sinks with draining boards are over £100, and a toilet will set you back another £300 or so for the cassette type. I recently had to buy a replacement water pump and pair of taps and the cost was over £100. Household type of units are too bulky and heavy, so specialised units are required and, as you know, anything specialised cost more than average.
Every commercial vehicle has a Gross Weight imprinted on a plate. That weight must not be exceeded or you will fall foul of the Law and probably your insurance would be invalid. The gross weight includes all fittings, passengers, plus any water, gas, food and personal effects, as in kerbside weight. Some older commercially-built 'vans are very close to their Gross Weight when only driver, passenger and minimal equipment are carried. Add in the two extra passengers for which there are seats and beds and those vehicles could be taken off the road at any time, should there be a roadside weight check. Motorcaravans are often pulled in with commercial vehicles by DVLA inspection teams.
To keep within this weight and to keep the balance of the vehicle furniture needs to be properly placed side-to-side and front-to-back, with heavy weights such as the fridge over the rear wheels.
Timber, ply and chipboard from the local D I Y shed is far too heavy and particular framing timber and lightweight board materials need to be used. Cupboard doors, for instance, are made hollow to reduce weight.
You would need the skills of carpenter, auto-electrician, gas fitter, plumber, welder, panel beater, mechanic to name but a few.
I believe there are likely to be laws introduced which will require motorcaravans to pass a "fitness test" before they can be licenced by the DVLA but I don't know how far advanced are those proposals.
If you are still considering doing your own conversion then think very carefully. You would probably spent more that buying a fairly new commercial motorcaravan.
Colyn.