A lot of people remove the belly pans when doing a resto or repair, or even when they're getting worried that they might have a bit of rot on the bottom... or if they're fitting a water tank... Etc etc. They can be a water trap and they stop you seeing what the underside of your van is really like.
My advice, take it into account but don't be afraid if there isn't one there. Just make sure the chassis, floor, sills aren't rotten, and factor in about £200 to buy and have one fitted if you want to.