Im with joyenes on this one.
We purchased an 8X10 Cargo Mate, single axel this spring. We ordered it with barn doors, 2 windows (one on each side) a light and a sky light. We painted it with a sealer(inside), added mats to the floor and made stalls. We are also adding two windows on the doors. For us, money wasn't the real problem, it was the size of the trailer, weight of the trailer and where we store it. We needed something light and small that would fit in our backyard and we don't even own an acre. We don't travel long distances so it didn't have to be top of the line.
I recommend getting an 8X10 or larger. If you are going to use it for long trips like, 5 hrs + one way i recommend a double axel. If your just using it for short trips then a single axel is fine. If you have a single axel and your driving with 3 minis it won't be that bumpy.
We looked at one mini trailer when we were searching, i don't remember the company name but they make regualr sized horse trailers but it had a single axel, came with no windows, wood floor, and barn doors and that was $5500. It was exacly the same as a cargo trailer but priced MUCH more. We got ours for $3500, windows, skylight, light so it was much more reasonable.
Trailer 1
Trailer 2
TRailer 3