Why do you want a van ?
What is it that you need from it that you cant achieve with a horse trailer + suitable towing vehicle ?
Its something I looked at quite seriously a while back. At the time, we had a large 4x4 + ifor trailer. The idea behind looking at a van was that it would give use somewhere usable as a sitting space for the longer times we spent at events and various wastes of hours. When it came down to it, we found that a lot of sub 3.5t van setups didn't really offer much of a usable sitting space etc ... in fact in reality, a lot less comfortable space than you really had with the 4x4. Then there was the weight issues as mentioned, and a lot of smaller setups may skimp on what could be serious issues, such as strengthened bulkheads. ( In a crash, you dont want 600+kg of mass going straight through the bulkhead. ) The 3.5t market just didn't really do it for me. I felt safer with the 4x4 + trailer.
The weight thing had me looking at 7.5t small lorries instead. There is a reason they are so common ... you get decent space with them. Trouble it its easy to go too big, a fault my friends did.
The suggestions of a larger van is what I personally had in mind at the time. Something like an Iveco 6.5t daily, which could pull double duty as a horse van and maybe something like a camper if needed. Never got round to it though due to horse injury.