Watch out for the older coleman popups with ABS roofs, they sag, they crack.
Don't think they have been made for several years now, but I'd stay away
Also with any of them watch for signs of leaking.
I had a 10ft box with front trunk.
I really wanted the front trunk for storage, but later decided that I was towing around a 12-13 foot box with the inside space of a 10. I could have used a bigger box on rainy days when the family would get stuck inside. The storage trunk was handy, but I'd prefer a standard box as long as it has some outside storage for hoses and a few dirty things. You don't need much.
I enjoyed our rockwood by forest river... until I stopped looking at it as an upgraded tent and started thinking of it as an RV. It was a good unit though
Pretty much any brand is good, they are all very similar, with the major components all being third party... things such as the fridge, the AC, the lights.... so it's mostly about finding one you like and one in good condition.
Look for one with a fridge that you can get to to stock while the top is down. That was probably my biggest beef with the one I had.
Also, this is a great forum, with nice and knowledgable folks..... many of us have probably owned popups in the past, but it seems most here are travel trailer and motor home folks now. You might find more traffic over at rvitch.com, or one of the other popup specific sites.
Good luck with your shopping!