This is what I would do if it was mine, based on 40+ years of trailering boats between 14 and 28'.
The master cylinder, by virtue of having rust on the inside, as you said, needs replacing. They can be rebuilt depending on how rusted and the availibility of the parts (kit).
Steel lines are always a P.I.T.A. I would replce all with the newer style nylon lines. They should last 15-20 years. And, not being steel, they CAN'T rust. Use care when tightening them, They are not as strong as the steel fitings; they are brass. Using a gorilla grip can split the fitting and it will leak.
Wheel cylinders. Most likely OK. Since the trailer was not used, there was no water being pumped into them.
The brakes themselves. I would pull the wheels/hub/drum assemblies, inspect and repack the bearings.
Fluids. non-synthetic brake fluid is hydrogopic. That means it absorbs water (or water vaopr). Moreover, the water mixes wit it. This is what rusts the innards of a brake system. The are also rated for higher temperature operation, meaning it don't boil at temperatures which the old one would. And, it isn't a lot more money for it.
Once you bleed the system, close all the bleeder screws, and pull the breakaway cable to put pressure on the system. Leave it that way for 5 minutes. If the piston in the master retracts, you have a leak. The you can look for it by eye.
