In 'Software Engineering' there is this thing called 'code reuse'. The idea being that you don't reinvent the wheel. We were all taught it (and moved on to realise there is a lot of code out there that is best not 're-used').

These Ducato vans are the popular 'platform' for camper vans in Europe, due to the low floor. Most of the conversions have worked out the hard problems and been 'debugged' with customer feedback. The swivel seats come already done properly with no hammer needed. The various tanks for various fluids are worked out. Spares can be bought, plus you get something better than really heavy plywood carpentry - actual cabinet-making.

We all have our own dreams, however, from my experience of living with a camper-van mad family, second hand is good. I would not go for brand new van that depreciates and custom prototype carpentry when people vastly more experienced than me have done it all before hundreds of times over.