Rule of thumbs for trailer shopping:
-Add up how many horses you normally intend to haul then add a spot, so in your case get a 3 horse. Extra slot is for hauling your third horse that you just haven't got around to admitting your going to buy yet or a friends horse.
-Check the axle ratings on a trailer plus the trailers GVW before buying. If they match or are close, don't buy it. Many manufacturers go light on axles to save money and that's not really a good thing. Running them at max or near max weight will decrease longevity and can cause a nasty wreck with precious cargo. You want axles under that trailer that are designed to haul a heck of a lot more than the trailer is rated for. IE trailer max GVW is 7000 pounds and the GVW of the axles is 3500 pounds each, walk away. Here's why. A 3 horse steel bumper pull trailer will weigh ~3500 pounds empty, figure ~1000 pounds per horse and you are at 6500 pounds with just your horses in it. Now toss in tack and supplies and you are at or above 7000 pounds.
-Check how much weight the trailers tires are designed to carry. Again many people and manufactures will put on lighter tires than needed to save money. Buy tires that will carry what the axle can at a minimum. IE if you have a 3500 pound rated axle the tires need to carry at least 1750 pounds of weight each. But you should apply the same rule as above, you really want tires that can handle more weight than the axle is rated for.
-Ask how old the tires are, if they are more than 5 years figure in replacing them. Trailer tires usually will blow out long before the tread looks bad on them due to sun rot. That makes age a better indicator than tire wear.
-Buy a white trailer, they are cooler inside than colored ones. Don't believe me? Go to a trailer lot on a sunny day, pick several colors then step in the back of them. You'll be able to feel the temperature differences. As you are moving to Arizona where it can spend months at a time over 100F, it will make a huge difference for your horses.
Now other advice.
Slant vs. straight is always a debate. I personally think horse haul better in a slant while others say straight. I also think they are easier to get a horse to load into as they are more open and inviting but that's my opinion. Go for what you want.
Get a tack room water tank installed. Handy for both long haul water supply and watering your horses in dry climates after a ride.
Canvas corner feeder for slant loads are handy. Get ones that are water proof and you can put water and hay in at the same time. I'm sure there's something similar for straight loads, just haven't looked.