vehichle insurance banding

[S.P.I.D.E.R]

I have to change vehichles as i have a disability and need an automatic camper or ambulance. Not sure how the insurance things are dealt with these days, can someone explain please? is it the engine size, type of oil? (as i would like a deisel, even better if it would run on bio!) are there other considerations and how do i work it all out? Im trying to keep the costs down as much as possible? any good ideas very appreciated.