Dsling;
Thanks for the clarification. In your budget range there isn't a lot of protective difference between the brands. Shop for the largest size you want with the features you need. Yes, heavier sheet metal is better than a lighter gauge sheet metal, but the actual protective difference isn't all that much.
If you can, find a unit with a plate steel frame. That may be impossible these days, but Frontier (now out of business I'm pretty sure) used to offer RSC's with that feature. Otherwise you're going to see the body sheet metal bent several times in the door jamb area & the bolts locking up behind bent sheet metal. And, as we all should know, bending sheet metal doesn't do anything good for its strength
The thermal protection values are going to be expressed as a time/temp figure. But, the numbers themselves mean nothing until and unless you know exactly how the test was conducted. You'll run across several different testers, examples being: Pyro 3000, Omega Laboratories, BTU test, and for all I know, Uncle Fred's Supreme Wow test. Underwriter's Laboratories are the only organization that actually publishes their test procedures, you can go online & find them. None of the others that I know of do that. Without knowing the test itself, the figures given are nothing more than advertising puffery. Beware of U.L. stickers applied next to the thermal numbers, read the fine print. The sticker will probably certify that the unit has passed the RSC test, or has a U.L. certified lock, but does not certify for passing the U.L. thermal test.
Good luck, the market is a jungle and there are a lot of predator's in it.
900F
