Realizing our opinions are jsut that, opinions.....
1. Your truck will easily handle a camper up to 4000lbs or so with the few mods you have.
2. I’d be looking at 8-9’ long campers. You don’t need a huge amount of room and with a long bed you’ll have hardly any overhang and decent weight distribution. IE you may actually get some of the weight on the front axle.
3. Brand and budget? I’ve only had 2 AF campers. Both have been pretty trouble free and I’d expect good service out of a well cared for model like you’re suggesting.
That said, I believe most brands would be the same l, as in reliability based on how they were maintained.