Maybe a safer way is to break the conversion down, in your case maybe into 3..... 3 parts at 5k each... Then pay each one, when completed on credit card, clearing the credit card balance from your normal bank account to avoid charges...
paying on credit card gives you, 'the buyer' more security as if you fail to get the 'goods' then the credit card company will recover the costs and reimburse you. It may not help if your van is in an unfinished state, but you would get the money back you have given them.
I don't know anyone that's done this with a conversion, but it would be possible, providing the converter is happy to itemise, the 3 portions of your conversion...
normally though, people do their homework and put their trust in the conversion company when and if they feel it's safe to.
Your insurance still covers you against fire and theft if say there workshop burnt down, and they should also have their own business insurance if such an unfortunate event happened.