Maintained while in service, not so much at end of life before sale. These trucks usually have interstellar mileage and even if it has a southern license plate, they see work up north in the salt. I bought a 89 Ford diesel box van to use for storage, the box wax pretty good, truck beyond bad. ( bought from someone that had bought from U haul and used it a few years )

U haul no longer get diesels from what I've seen, I bet they figured out that the extra purchase cost / repairs isn't offset by the slightly longer engine life. ( U haul gets no benefit from reduced cost per mile of fuel use since the customer pays for fuel. )

If you are near a military base / state maint garage / university, there might be some stuff at auction. Have a look at www.govdeals.com
and their related sites, they seem to be a real auction site not a " Seized Mercedes SLR for $ 500" type site.

A trailer or box truck converted to a trailer would be the most direct route but trailers have a pretty high theft rate. A box truck / smaller U haul converted to a trailer is a bit more conspicuous and homemade looking so it might not attract much attention of thieves. I'd go so far as to sink 4 anchors into the ground and chain the trailer down, add some sort of lock to keep wheels from turning and a motion detecting / door open alarm system. If you do the converted box truck the parking cables can be used as a wheel lock.

To convert, cut the frame under the cab, fold inwards, add a hitch coupler. Hydraulic brakes can be operated my a surge brake style coupler.

For what you are doing, a box van will end up being more expensive to maintain. A used ambulance is a possibility. However, given a limited budget, the best choice is that combination of purchase price and running costs. If someone gave you a box van , that sure would tip the scales in that direction.