Probably the best time to look at a used box is right after a heavy rain. Most carriers will require the box to be weather tight, so after it rains look inside for any signs of water leakage. Check the floor boards for any damage, and if you can look underneath, make sure there are no cracked or damaged support members. Also make sure the floor wood isn't rotted above the area where the old truck's rear wheels were below it. If it has rollup door, check the bottom panel to be sure it isn't rotted, and make sure the lift springs operate in a decent manner- don't expect perfection here, they require a lot of maintenance to operate smoothly. Check the hinges if barn doors, and the weatherstripping on either. Boxes made by Morgan or Supreme tend to be recognized as the leaders. I found the Babb box on my old truck was decent too. I have a friend with a Kidron box, who says it does nothing but leak, might want to avoid that make. Not a durability issue, but a box that is lined with e-track installed will be to your advantage when looking, as many carriers require this. If the exterior of the box isn't all banged up and the price plus all the above is good, you have yourself a box. For expediting, the "bigger is better" saying reigns true.
-Weave-
Sounds like good timing to me, although it's not a terribly hard rain, it is raining.