Whether building a new trailer or rebuilding an old one, it all depends on what you want to put into it.
You can go more "deluxe" when you are doing the work. An example is 14ga diamondplate fenders. They aren't as cheap as what the factory uses, but they won't bend when you step on them and you won't slip. You can make the steps larger, and stronger with aluminum diamondplate covers that work better than factory trailer steps, but are too costly when building "factory" trailers.
You can use stainless steel hardware and bolts, where most factory builders won't spend the money.
You can use Stoltz rollers and stainless steel pins. They aren't cheap.
You can use nicer wheels than most factory trailers come with, but that adds to the cost.
Many boat trailers come with cheap couplers, especially the ones with no brakes. Collar Locks aren't cheap.
When finishing a boat trailer, I would rate the finishing options as follows.
The least expensive, but adequate is a good primer and paint. Doesn't last as long as other more expensive options.
Better is powder coating, but it costs more than paint. It looks better than galvanizing but isn't as durable.
The best is hot dipped galvanizing, however it is as expensive as powder coating, doesn't look as good, but it is the best for corrosion resistance. Some of the best steel boat trailers will be hot dipped galvanized.
The disadvantage when rebuilding a trailer over building a new one is the metal preparation. If you are painting, then a wire brush on a grinder will work. When powder coating or hot dip galvanizing, you must use a sand blaster to remove all the old finish, even in hard to get to areas.
Buying a used (newer) trailer sometimes isn't as good of a deal as rebuilding the older, original trailer. The trailers that come with a particular boat are usually built for that size and shape boat. The length, width and roller or bunk locations etc. work with that particular boat design. There is a lot of latitude available, so swapping trailers usually isn't a bid deal, as long as you are within reasonable parameters. I've seen people buy a donor boat and trailer mainly for the engine and outdrive. They decide to use the "donor" trailer because it has a better paint job. When looking at both trailers, having a background in building them, I've seen where the owner would have been better off rebuilding the original trailer since the donor trailer was close to needing a rebuild in the near future and it didn't fit the boat as well as the original trailer did.
You can't compare a "deluxe" rebuild with a "standard" grade factory trailer. The manufacturers usually don't put the extra effort into trailers that are mass produced. My last boat trailer rebuild I did, I rewelded the entire frame. The original MIG welds were adequate, but nothing to brag about. You won't find fancy removable side rails with receiver tubes on regular factory trailers. Manufacturers won't spend the money and time.
My suggestion is to go with the least expensive options if you aren't sure you are keeping the boat and trailer for years to come. For me, I went the expensive route because I was planning on keeping my boat for decades and I only wanted to do it once.
