Hi, It is a tricky one, also down to what you can afford, when i had a LWB transit i Had 25,000 git on it but never really had a load that would require that, know i have a Nissan Primastar so i have 10,000 with the option to upgrade if required, which i have once so far when carrying a picture with a 60k price tag on it, but when i quoted to move this the extra insurance cover was included in the price.
I Personally think you should chat with your Insurance Company and explain the situation and see what they suggest.
I cant see the point in having that level of cover until / if you get some work with them, and if it become a regular contract then be sure to price the extra Insurance into the quote otherwise it wont be worth doing.