I bought a used 28'.
What was posted earlier is really good.
At the time the #1 wish was the large drivers side escape door. Otherwise I would have to winch the car in and out.
The trailer came with the normal 5500# axles and 15" wheels. Don't even bother with the 15" wheels, the tires won't last long if you have any weight in the trailer. I replaced the 15" wheels with 16" wheels and "E" load range tires, price was not too bad, so if you find a used trailer with 15" rims, they can be changed out. Also, as mentioned it would be nice to have the higher rated axles.
My used trailer has the 12v/110v light package, and the exterior lights, plus the roof mounted A/C, but no generator compartment. I don't own a generator, so I have never used the A/C or 110 lights. As far as with list goes, if you want those options, it would be easier to have the builder wire them, then pull everything apart it wire it later.

Along with lighting, extra side marker lights around the wheel flares.
My trailer originally just had reflectors, but the lights are nice when backing up at night.

A place for the spare tire. My trailer did not come with a spare tire compartment or any place to store the spare, so it can get in the way. I did mount "E" track to some of the walls, so I can strap down items to the walls, and I have the spare strapped to the wall that way.

Metal roof vents! A hail storm took out the plastic roof vents in my trailer and the Charger inside got hail damage on the trunk and hood. The storm also destroyed the roof mounted A/C unit.

My trailer came with cabinets, upper and lower across the front (no side/generator), and they are OK, but not great for accessing stuff.
If you have items in front of the lower cabinets, you can't open them.
Really some storage racks would hold the jack stands and other stuff I have in the cabinets and be easier to access. I use the cabinet top quite a bit, but a folding table top would work too. Anyhow, depending on cost, I might skip the cabinets and just go with storage racks?

I'm not sure what everyone is using for the backup break-away battery. The trailers come with them, but my battery seems to need replacing every other year. I have been thinking of just wiring a regular car battery inside, under the cabinets, and adding a decent remote control wench for when the car breaks. Currently, I just use the cheap HF boat wench that I got on one of their sales. It works, but is super slow.