I've owned two T4 Cali's my first was similar to the white one - mine still had the sprit burners in it and the 2.4D engine, which was an excellent runner, but made long journeys tiring as it just didn't have the kick. I then went to a later one with the 2.5Tdi engine and that was far better.
Price wise the white one seems well priced if you can put up with the slower engine and don't mind doing a little bodywork at some point. Once the season starts properly I would be surprised if the white one didn't make £10k - so adding the heater you want, would suggest you would get your money back.
The later T4's are making very strong money and although I loved mine, the interior units were a bit dated. I sold mine in July for about £15k (3 years ago) and bought my T5 Cali in Germany for about £22k (2 years ago when the exchange rate was better) - but it felt quite a big move up in terms of refinement and looks.
To be fair joining the "club" and becoming a Cali owner comes at a bigger cost (compared to a conversion), but for earlier models of both T4 and T5 Cali, the increasing list price of T6's keeps dragging the prices up. So my advice is spend the money and get what you really want. If you keep it for a few years you will always get your investment back and also possibly make a bit. The logic with California's is totally different to any other "car" you will have bought - with the exception perhaps of a classic.
Good luck whatever you decide.