Having used a motorcaravan as my only vehicle for in excess of 25 years perhaps I can offer some advice.

Firstly, look at as many different types that you can find. A conversion of a Transit-sized van you may feel is somewhat cramped for a family of four, although as was said earlier an awning for additional sleeping is not out of the question.

A small coachbuilt van may be more advantageous, usually with a double bed above the cab for the children (or in my case, the dogs) and either twin or double bed below. Coachbuilts usually have a larger galley and a shower room. The main disadvantage is their size, although this can be an advantage when moving house.

With all campers, watch out for height barriers!

Also be aware of the maximum load capacity. The maximum load, now called something like Maximum Gross Vehicle Weight, is on the chassis plate. It includes occupants, fixtures and fittings, gas, water, fuel. Any allowance left over after having the vehicle put on a weighbridge can be used for food and luggage. It could be that a conversion may leave you, after your wife has loaded her own holiday wardrobe plus that of the children, with only an allowance for one clean shirt and a pair of socks for yourself!
The Stazi, sorry, the Police are becoming much more prone to stopping campers and taking them off the road if overweight. They are more hot on the older vehicles.

Be wary of the advice about DIY as many insurance companies will not insure DIY conversions. Also the neccessary gas installation for cooker, fridge, and possibly heate, must be installed by a registered gas fitter (late CORGI, now something else). I have done a total rebuild of a coachbuilt van and one needs to be adept at mechanics, welding, panel-beating, woodwork, upholstery, auto-electrics, domestic electrics if fitted to van, plumbing, as well as inumerable other skills.
Also one needs access to a supply of specialist lightweight plywood, wallboard, etc as bog-standard ply, MDF or chipboard from B & Q is far too heavy. Your weight limit would be exceeded before you loaded yourselves!

Joining the Camping and Caravanning Club, about £35 p.a., is a good idea, as they have a wealth of information. The cost is saved during the year by reductions on site fees.

If you would like to PM me with your phone number I will happily give you what advice I can.

Colyn.