Got any pics? The vehicle being registered as a box van but there are other issues…
The big thing to get with these luton van conversions is the headroom for the horse…they can be a bit tight although removal vans are often higher.
Check the ramp for steepness, bendiness and size of hinges. Check the gap between ramp and body, I witnessed a horrible scene involving a converted box, the horse stepped on the gap and forced its foot through, the whole show got cancelled with a crowd of people panicking, eventually ending with the fire service and a vet to do the inevitable as the horse had torn its leg up.
A proper 3.5t van conversion will often have a payload nearer 1500-1600kg…also have you considered a trailer. We are out most weekends until recently and use a 3t Equitrek trailer which has a very low ramp and feels like a lorry for the horse. We’ve had it years and done tens of thousands of miles in it, accomodation for weekends away (including biking) is sorted by pulling it (or not, if biking) behind a campervan!