certain trailers are made with a bit more focus on quality and longevity as well. in my neck of the woods, the H&H brand of trailers had robust, low temp, rubber sealed wiring harnesses that every connection was in a true trailer type sealed junction box, or was soldered and heat shrinked to prevent water ingresss and future corrosion issues. CV trailers and Snake river trailers were considerably cheaper to purchase, but they used the run of the mill supercheap plastic encased wiring that is a little four coloured band of wires that break when moved or jostled in cold weather, splice connections were those junky blue splicing things that just crimp through the insulation of the existing wire and add another wire to the circuit, light terminations were blue, plastic crimp on butt connectors with no tape or sealant to prevent water ingression. i purchased a brand new CV trailer because of the price difference, i figured i could entirely rewire the trailer for less than the added cost of the H&H trailer where the only real difference i could find was the wiring system. the original wire harness lasted thrtee years and i did end up completely re-wiring it for about 150 bucks. if i had to pay the labour that cost would have easily been over 1000 dollars, and probably wouldn't have been to the same quality or all new materials, they would have just patched the wiring where necessary.
my new snake river trailer has the same issue as the cv trailer, poor wiring harness, it is still functioning so i will fix it when it fails.
another thing i do that i have seen many other people do is add extra mudflaps under the body of the trailer to prevent the rock damage to the electrical system.
dexter axles are pretty much standard issue in canada, and parts are cheap and readily available. it is cheaper to replace the whole backing plate and shoes with hardware, springs and a new magnet then it is to buy the shoes and the magnet and the springs separatley. there are other axle manufacturers out there, but i have found that dexters are most readily serviceable in any small town. you can almost always find a shop that carries a backing plate or two, a spare rim if needed, studs, nuts and hubs, and wheel bearings. you may not be able to rebuild 4 or six complete hubs, but if you lose one or two you can usually be back on the road in a few hours.
i have also seen a lot of frames that have mud/dirt sitting in the inboard side of the main frame i beam, this can cause excessive corrosion as it never gets washed out, and the sun doesn't shine on it to dry it quickly. check the frames for corrosion if buying used as they are generally never painted on the parts that no one can see from walking around the outside.
i don't have experience with owning hydraulic surge, electric over hydraulic, or vacuum over hydraulic brakes. my dad has a vacuum over hydraulic trailer that is a pain to use as none of the trucks he has anymore has the vacuum controls. i beleive he converted that trailer to pure electric brakes, it's been about 15 years since i have towed that trailer so i don't know what it's current condition is.