Hey welcome aboard!! So... you're talking about a Regular Cab/Short Box truck, right? 2-WD or 4x4?
Pretty much what you listed off are the main things. Exhaust manifold warpage/leaks & broken studs seem to happen in about the range you're looking at. It's not a small job (nor cheap) to fix, so you want to pay attention to that. If you found a good prospective truck you were willing to buy, you could pass the VIN to a chrysler dealer and see if there is a record of the exh manifold's having been repaired. Engine-wise, these seem to be pretty good. There guys like me who have a boatload of miles on our Hemi's. Hemi tick ..you might as well assume your truck will have it or develop it. If you can't get past that, the Ram might not be the truck for you.
Rust-wise ...you live in TX ...what are you worried about?? LOL... But look at the usual things like rust on the frame/bolts/diff, etc. If they're real clean, then it's a good bet the rest is too. But if you DO buy a pickup, I would remove the rear plastic inner fender liners (if your newly bought truck has them) and hose out the wheel wells. Or clean them. They can fill up with rocks dust and debris. I don't know that I would do any undercoating in TX. Anything you apply above the wheel arches is just going to "hold" crap/crud and make it harder to wash out in the future. Esp if it's 'gummy' like most of the undercoating crap is. Applying some oil at the bottom of the tailgate might not hurt either. IDK if that's even necessary in Tx.
The outer drivers seat is a good wear indicator too. They often get boogered up b/c the truck is taller & there's a seat frame member right under the outer seat lip, which pinches the foam as people slide in/out like it's a kid's slide. If you found a truck with a worn seat, you might negotiate in a new seat cushion and fabric/leather ....if it needed. If the fabric is still good, a new cushion might be enough. But then what brand cushion?? (that's another story altogether!).
A warranty might not be a bad idea depending on your penchant for risk or ability to pay for repairs.
Signs of flood damage are going would be water/dirt/mud up high (spread evenly) ...but most sellers would probably clean it up so you'd have to remove lift up the mat/carpet or stick your fingers (or inspection cam) into the frame. Hard to detect those things via online pictures if it's been cleaned up. Check the paint work for signs of refinishing (hail damage?), etc. If it's been done 'right' that's probably not a problem.
Have new truck inspected on a hoist by a 'good' shop prior to purchase is a good idea. Overall these trucks are pretty solid, but any time you have 20k parts screwed together bouncing down roads ...well, you get the picture, things can go wrong.
Good luck
