If demand is still high the they are less likely to accept offers, if the layout and configuration is rare then even more so. I don’t know what the construction of this van is but one like mine with sand which construction leaks wouldn’t be a big issue. If it’s wooden framed then it’s more of a problem. As Trev has said (and shown) repairing the floor isn’t hard. If it’s in the garage area you should be able to cut and replace as needed as no cabinets to mess around with?
Depends how much you want that particular van. Can I ask, why are you so worried about the mileage? Personally, so long as it’s been serviced I wouldn’t worry about a higher mileage, my van has now done around 105k and sweet as a nut (hole I haven’t just jinxed it). My thoughts are it’s preferable for a vehicle to be used than sat around, it doesn’t do motors a lot of good sitting.