Some trailer websites like Load Rite have information on how to measure the boat to determine the right size trailer. Thats the easy part the hard part is estimating weight since they always weigh more than what the specs say. Usually you have to know the beam at the widest point and the bow eye to transom measurement, this information will often be on the boat manufacturers website. When you talk about trailer weight capacity keep in mind that the GVWR of the trailer is not the same as carrying capacity. The carrying capacity is the Gross Vehicle Weight Rating (the most it can weigh all in) minus the weight of the trailer.
Having maintained a salt water trailer for years, the things that will give the most trouble are the brakes and lights. Brakes, if you can get the best ones possible, stainless discs. I have drums but my boat sits on a mooring so it doesn't get dunked more than 4 times a season and I have learned how to modify the wheel cylinders to keep the salt water out which helps make them last. The other is lights, whether you have conventional or LED lights, the grounding can be a problem. The way to stop having issues is to run additional ground straps, either to each lamp if you are really compulsive, or if you are a bit lazy like me and run ground straps from the tongue where the harness ground is to each leg of the frame and then to the tail light mounts. Doing this greatly reduced my lighting issues.