I just got a pair of ST225 75R15's for our trailer. They were load range C, which is what the trailer came with, and are within 1/2" (maybe less) difference in diameter and width of the old number series tires. There seem to be many opinions about load range, my opinion is to use what the trailer came with. If for some reason you want to go with a heavier load range, I again would reccommend the stock inflation, which on mine is 50 psi. The more air you put in the tires, the rougher the ride for the trailer and everything in it.
It is also a very good idea to balance the tires when you get them, to keep the shaking to a minimum, very important on an A/S.
We just went on a 1800+ mile trip with ours, with the new tires, no problems at all, and when we got back here, I looked at the tread, and the little casting seam is still visible in the center of the tread (a good thing).
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.