My dad and I have built a couple completely from scratch. I usually find it cheaper to modify the one that comes with the boat (why can boats not come with a trailer that actually fits), than it is to build a new one, but on some trailers, that isn't possible, as what you have to start with is not really workable.
It isn't hard at all. In fact, it is downright easy. Buy your axle, springs, wheels, hubs etc from Northern or Tractor supply, and then get steel from a local steelyard. Really one of the easiest things you can make with a welder (for the most part, it is one dimensional, and as long as you keep everything plumb, you are set).
Ben, I have to respectfully disagree on those HF and Northern boat trailers fitting a 16 foot boat. They trailer, from coupler to back of the bunks is only 12.8 feet long. That is a good fit for a 10 foot boat, and a tight squeeze for a 12. It will not fit a 14, and the 14 will hang off so much in the back that you will probably see a hook develop in the hull. I wouldn't even try a 15 or 16 on it.
What I have done successfully, is take a Northern tool 42 x 48 inch trailer frame, and do a little modification on it, and it turns into a perfect fit for a 1436.
Here is one I did recently for a guy that had me do a little work on a 1436.
https://www.shareaproject.com/pages/projectTut,p,528,00.html
But, I don't think I would go that route on a boat much bigger than a 1436 aluminum. For the Whaler in question, if you can't find a good used one, building one is a great alternative, and while costly, you have brand new everythings, so you know the history of everything, and it will fit the boat perfectly, and suite your launching needs. That is my biggest issue. I am much more picky than most on my trailer, and rarely can I find one that fits the boat the way I think it should, and thus need to make my own.