We had bought our first rv as a used pop-up. Palomino. Didn't think we needed AC and the price was good. It was fine for traveling in the maritimes. Then we decided to take a long road trip to virginia beach and stay some time in Washington, D.C. . Realized that we would need a AC united for that trip. Well buying a rv AC unit is ridiculously expensive. Looked at lots of mods that other people have done. My DH didn't want to cut into the trailer box for these mods. I ended up bringing our portable AC from home. On it's back, I could store it in the popup for travel. At the campsites, I could put it up on cabinet by the door. Just vented into a window screen and zipped the vinyl window around it. Worked pretty well. Could get the popup down to about 82*f on the over 100*. Days.
I made sure that our next rv (a HTT) had AC. So, yes given the opportunity, I would suggest buying a popup that has one already, even if you end up only needing it once... But I wouldn't spend over $450 more for it, because you can buy a portable AC for less than that, if needed.