If you price up a T30 Highline panel van on the configurator with as much of the same van spec as a Cali Ocean, the difference in price is about £18k....you be hard pushed to get a quality conversion with aircon in the back and camping tables/chairs etc for that and you’d still end up with a van plastered internally with carpet, a big hole cut in the roof and an no 3 year warranty on it all.......
I think it's unlikely you can save any money basing a conversion on a new van with similar spec as a Cali, if you want the same quality camper. Something has to give, either a lower spec van or a used van of unknown origin or a lower spec camper or simpler camper.
Having looked at several conversions, my son in-law has a T5 LWB beach style with pop-top, its good enough for weekend warriors, I would stick to a California "any California" whatever fit's the budget...
As to the bit about "a big hole cut in the roof" well! thats exactly what VW does! any competent camper builder will use the kit that VW supplies its riveted and Sika'd to the body and restores the integrity to the van body, there are some none VW kits available that should be avoided as to unknown design and metal strength.
A bigger issue is holes cut in the van for windows, VW has strict instructions as to reinforcement needed to retain body integrity in an accident, some is as simple as Sike'ing existing internal ribs to the outer panels, some is a bit more complicated, in many cases these requirements are not followed.
As to me, I'm sticking with my 2000 T4 2.5 Tdi, solid build and stood the test of time, it's indestructible as is the Audi 5 pot, yes its a bit like driving a truck, suites me fine been around trucks all my life.
If I change it's coz the boss want a higher van with a toilet, I'm digging in... if I do it will be self build, or maybe compromise and put on a high-top, Reimo is looking favourite.