You definetely can buy a stock trailer with a seperate, enclosed dressing room in the front. Some stock trailers are called "combos", meaning they can be used as either a horse trailer or stock trailer. I would check out those before going further.

I've used stock trailers for many years, and they have their pros and cons compared to horse trailers. I'm just curious - and not trying to change your minds - on why you want a stock trailer for horses compared to a horse trailer. I've used both, and to my surprise, my horses seem to ride more relaxed and comfortable in my Featherlite horse trailer as compared to how they rode in my stock trailers. I think the main difference is - my horse trailer is quite a bit quieter, and has better shocks so it rides smoother. So if you decide to get a stock trailer, make sure that you check to see how noisy it is compared to a horse trailer. For example, even the Featherlite sales person told me that Featherlite's stock trailers tend to be noisy, at least compared to the horse trailers.

Another feature I'd want is the ability to close the "open slats" that a stock trailer has. Without this feature you're stuck when it's raining, snowing, really windy, too cold, or even just down-right dusty. I've hauled horses to 4-h and other activities in the winter, and no matter how heavily I blanketed them, I think the trip must have been less than ideal in the stock trailer! Some stocks have good quality slider windows, I would insist on that if I went that route.

Having grown up with stock trailers, I always thought that horses would ride more comfortably in them compared to the confined spaces in a horse trailer, but that hasn't been the case. My horses seldom would put their heads down and eat when hauling in the stock trailer. I don't know why. As I mentioned, maybe it was the noise (from the trailer itself banging more than the outside road noise) or maybe it was something else. But if I put hay on the floor of the stock trailer, often I would arrive at my destination to find they'd hardly eaten anything. In the horse trailer I feed of the floor (I have a slant load, and I let each horse ride loose in their slant) and they eat the entire way. To me that indicates they're more relaxed and comfortable, even though they didn't get to choose the exact position they stand in.

I think stock trailers can be great - they allow flexible positions and riding options. But I think you need to try out ones first and see how they work in real life, and how comfortable your horse seems in one compared to the other options out there. Happy shopping.