Dropping the floor typically requires major modifications to the frame of vehicle, often results in considerable amount of reinforcements to the frame. This is going to put you in the same place as a customized van with reduced cargo capacity.
Raising the roof of the van, typically they will cut off the roof of the van then install a fiberglass roof which gives additional headroom. Fiberglass roofs do add considerable weight.
Not impossible, but this can be a substantial issue to take into consideration because you typically will run out of cargo capacity well before you hit the tow ratings.
Your hitch weight becomes cargo of your vehicle and with bumper pulls your hitch weight MUST be no less than 10% (10%-15% is typical TW range)of the trailer for a stable tow. The higher TW, the more stable the tow so ideally you really want to shoot for 15% TW when loaded.
15% of 10,000 lbs is 1,500 lbs TW without taking in account weight distribution hitch weight.
Additionally, your van may not have a hitch receiver from factory and adding that will reduce your available cargo.
For your needs, I highly recommend you look up custom horse trailer builders, I have one in my area that builds theirs out of all aluminum including the frame..
They can make them not look like horse trailers and should be able to build to your specific needs with wheel chair access designed in from the start.
Most likely they will be able to build it quicker and you won't have to spend so much time DIYing it.
Another option is to start with a cargo trailer but they tend to be pretty heavy to start with and that may become an issue.
