Thebogflogger, you need to decide what you are trying to achieve, I need to use my van for commercial use so I have commercial van insurance, it's more expensive than standard campervan insurance. If your having the work done by a campervan company they deal with the fit out & usually the change of usage on your v5 form to camper or car, in my case I am keeping the van as a van & am changing my v5 to show the vehicle now carries 6 people. The d.v.l.a have asked me to take pictures of the vehicle before & after fitout then send them in to have the v5 changed. You would still have to follow this procedure to have a van changed to campervan status. Seats as far as these are concerned the seats have been crash tested by ford, so there not your problem, getting them fitted is, there are loads of companies converting transit custom vans & getting them insured is not an issue, in my case I popped down to the local ford garaged & photograph how they did it, there reinforcement is pretty sparse, I opted to beef my seat reinforcement up & use 5mm plate steel, I also picked up some of the original nut positions already in the chassis. Of course if you unsure you can use aftermarket seat products like jany that are already have N ratings but I think the ford seats are better.
you will have to go for a specialist insurance though I found my insurance company had no problem with me putting seats in but the mere mention of putting windows in the van means they could not insure me. Work that out!