The California by VW is complete with a 3 yr Warranty from VW covering ALL aspects of the vehicle and Spare parts for the Habitation area are available at least back to 2005 or further. It is also registered with DVLA as a M1, Diesel Car - Body Type Campervan.
A Brand new Conversion will be based on a Commercial Van registered with DVLA N1, Diesel Van or equivalent . This CANNOT be changed, but the Body Type can be changed to Campervan if it fulfils the DVLA requirements.
A Brand new vehicle will all the driving extras that a California has will cost in excess of £40,000. The converter will then fit windows, a pop-up roof and bed a Bench seat, as the lower bed, that has not been properly crash tested and certified and kitchen and storage units made out of MDF or plywood. Also the walls will be covered in plywood and carpeted. A Mains and Leisure circuit with lights, charger, fridge, parking heater, split charge system all have to be added as well. Front seats fitted with swivels. Water and Waste tanks, Gas Lockerwould be fitted.
All in all a Conversion with all that the California has will cost more AND you will only have a limited 3 yr Warranty from VW on the base vehicle and a 1 yr Warranty on the Conversion.
Lastly Depreciation on a California is significantly less than a Conversion, in fact it is significantly less than almost any new vehicle you can buy today.
Insurance of a VW California is not a problem. Plenty of companies to choose from.
If finances are a factor then a conversion on a 2nd hand van can make sense but you will not get the same facilities and quality as a VW California without spending a lot of money .