I would agree with all the advice above.
I'm enchanted with the van conversions that folks here have done
. They all look totally cool!
We have 2 Minis. We found a 90's used 2-horse slant trailer in the local market that needed new wiring, lights, and some paint. But got it at a great price. I did all the work myself for around $500 total.
We took the divider out as it serves no purpose for our little guys.
The trailer has worked fantastic for us, and I've even taken "the boys" on a 1500-mile trip to go to horse school.
They loved it. Kept to their normal feeding routine and they did fantastic!
Albeit, I already had a truck to pull it with.
One of my concerns with considering just getting a non-animal smaller trailer is that they are not designed with any light or ventilation. So you'll need to modify any such trailer to add rubber mats on the floor, possibly side-wall protection, windows/something for light, and windows/something that can be opened for ventilation.
And, you'll need to make sure it is not so high off the ground that it is hard for the mini/pony to get in/out of.