Automatic Fly

NO one can give you a definite answer to your question.
Because a lot of what you ask depends on your location, which you have not given us.
Your first question would depend on taxes, insurance costs, utility costs they do differ.
your second question depends on what the trailer is made of, steele, fibreglass, aluminum. It makes a big difference on the cost as well as whether it will have LQ or any frills on it.
third question, most comon brands are good or they would not be in business long. Check with your local feed stores
fourth question...it does not really matter how tall your horses are, it costs about the same to put shoes on a 14HH or a 16hh horse. The difference in cost is who you get to do the shoeing, not every farrier charges the same.