If I recall the original post correctly, the poster already owns a nice big truck; why not just buy an inexpensive stock combo trailer, which has room for both horse(s) and cart, and will have a decent resale, as it is more versatile than something which can only be used for minis? The hauler pictured will cost as much as, if not more, that such a basic trailer, and likely not hold its resale value as well....
I have a 14' X 5' CM stock combo bumper pull; it pulls and rides (for the horses!)extremely well, and has been VERY servicable and easy to use. The only change I would make would be to get the 6' width. I can carry a driving horse and cart, or even, a pair of horses and a 4 wheeled vehicle-or, if just hauling miniature horses, it can comfortably haul several!
HG makes a VERY good point about safety in hauling. Having an animal of the weight of a miniature horse(one, or more) in the vehicle with you, or even in a pickup bed, is at the very LEAST going to create disconcerting weight shifts-esp. if, as is so often the case in modern day traffic, you have to hit your brakes,or take evasive action, due to the actions of other drivers....also, having the trailer entry doors blocked so that they cannot be opened immediately is IMO very dangerous, as being able to let the horses out immediately could be of prime importance! It is not the best idea to even have the cart loaded behind the horse(s), so that it must be unloaded before they can be accessed...in a stock combo trailer(and most others), you can get one or more 'escape'doors-designed so that you can lead a horse into a trailer and then get out 'up front', it works very well as a way to load/unload miniatures, if needed. Yes, it IS a 'step up'; I don't like ramps-think they can be less safe than a step up--and have loaded/unloaded minis of all sizes MANY times in my stock combo, with nary a bump(and of course, you can use leg wraps if you feel you must.) The best weight distribution is for the horse(s)to be up front(or more precisely, over the axles), but with miniatures in a trailer, this is not nearly as big an issue, and you can load the cart/carriage in front, the horse(s) behind(a stock combo even has a divider gate, so the horses can't get 'tangled up in' the driving vehicle), for safety and immediate access to the horses. BTW, it has been noted that horses will often ride backwards in a trailer, if free to choose. I personally believe that slant loading is HIGHLY overrated, as far as being the best choice for the horses' comfort...backwards, straight front, or even perpendicular to the direction of travel; or even slant--all can work IF the horse has adequate room to brace during travel. It should be remembered that an average adult miniature's poll, in normal stance, will be around 10-12" higher than its withers; having to stand 'hunched' in a pickup cap so as not to bump its head would NOT be very pleasant for the horse. Also, good points to remember about the suspension of most utility trailers NOT being adequate for hauling live weight(animals)...
JMHO, after a good many years of hauling horses....
Margo