I'd suggest getting a profesionally mounted rack installed on a minivan or full size van, with heavy plywood on the rack. I've got a 15 year old Aerostar, and when I climbed on top not long ago to change a carport light bulb that wasn't reachable with my step-stool (I weigh just over 200 lbs), I left quite a visible dent which, fortunately, I was able to pop out from inside.
Along with the roof rack, there are ladders (sold for RVs) that mount to a rear door; two rungs on the tailgate of my van would give me the boost I'd need to climb onto a roof platform.
However -- I don't think a vehicle of this type, with my weight (and that of a rack, platform, large format camera and tripod) on top would be stable enough for exposures longer than around 1/30; just my breathing and involuntary movements would move the platform. That would leave me wishing for a set of leveling jacks, of the kind found in motor homes and travel trailers, in order to steady the vehicle; by the time I have a set of those and use them at a stop, I could just as easily have set up a pair of A-frames and a wide scaffold board between them, at less cost and with less weight for the vehicle to haul around.