Are you using it for a trail rig or only a race car? A trail rig on 35's will barely fit into a standard height trailer.
Get it insulated and finished inside.
Get the 110V flourescant light package.
Decide where you want your work bench/ utilities and specify where you want your 110v plugs.
Get the RV door latch on your side door.
If you plan to tow it with an RV, get the extended tongue.
28' is super long unless you are planning on 2 cars, a car and a golf cart or sleeping quarters up front. I started with a 24' for my jeep and dropped down to a 20'. I've talked to people with sleeping quarters up front and they didn't like it because if they left their vehicle in the trailer while sleeping (rest area on expressway) there were always fuel fumes.
I would plan out a floorplan. My guess is you will find you don't need that much room.