Point 3 your right on. With a 250/2500 series you will be really surprised how little real cargo carrying capacity you have.
Northwood makes good but heavy fivers. I think if I were you I would start looking at shorter, lighter models from Forest River and Keystone. Models like Cougar are quite popular.
Point 2. Basicslly the exact same things you would need for a TT.
Point 1. Slider would be best, but depending on trailer choice you may or may not actually need one. Many newer fivers are cut specifically for short bed trucks. But dont hold your breath. Just get a slider hitch.
Point 4, shorter overall down the road length is the biggie.
Point 5, tou probsbly wont find auto leveling on any short fivers that will fit your size requirements. Besides, its no big deal as long as your reasonavly healthy to lower the stabilizers using a cordless drill.