Sounds like you have this handled at this point, so I'll just make some general comments for anyone considering doing the work in the future.
I'm in the process of replacing the brakes on my original 26X steel trailer, with a second axle added. I priced out the 2-axle drum brake kit from etrailer, which included a new coupler, lines, fittings, (4 each) drums, bearing and seal sets, and free-backing brake assemblies. About $400 complete, with free shipping.
A single-axle disc brake kit was about the same money, with the same coupler (disc brake master cylinder, though), and included a reversing solenoid (normally open, though), so I went with that. If I feel I need brakes on the other axle, I can add them later with an expansion kit. The 2-axle kit was $200 more, so $600 would have done it all, but blown my budget.
Parts to rebuild the surge coupler was about the same price as a new coupler, so no savings there. I did have to cut off the old coupler, as it was also welded, but I probably could dismantled the new coupler and put all the parts into the old housing and not messed with the welds. New parts separately, without drums, was more money than a kit with drums, so no savings by buying piecemeal.
A kit was the most economical way to go, in my case, and based on the strong recommendations for disc brakes these days (and some inspiration from Sumner's web pages), I decided to go that way. No more work to switch over to disc as compared to replacing the drum brakes in-kind, and I shouldn't have to worry about them for years to come. Hopefully.