Basic carpentry skills will certainly be an advantage here. Units can be made to measure in situ or built in a workshop or garage and then fitted in place. In the event you’re likely to do a bit of both.
Standard kitchen type units bought from a DIY store could be adapted to fit, but do be aware that these type of units are often constructed of heavy chip board and will eat into your ‘payload’. Framing can be built of softwood then clad in thin ply to save weight. Ply can have a basic finish or have a very nice veneer finish. Again, don’t forget to look through the pictures of members vans to see the differing methods that they have employed when building the furniture into their vans.
Lightweight board such as ‘Vohringer’ board, with an assortment of finishes and often used in factory built vans, can be bought from various suppliers. However, a low cost option it is not!
Of course if you go the ‘caravan donor’ route then you will have lightweight units to hand, although they will of course need a fair bit of adjusting to make them fit. It’s highly unlikely that you’ll have the same space in your van that there was available for those units in the donor caravan and the walls will almost certainly have a different profile.
DVLA requirements for a van to be classified as a ‘Motor caravan’