First, sorry if I end up telling you things you already know. I'm still just learning all this myself, and trying to pass on what I can.
Craigslist is blocked at work for me, but that first one looks really nice from the description and photos. I can't tell what axles those are. I run 36" Mickey Thompson tires that are super wide, and although I only have plans for driving on the street for now, I can tell the steering system hates them. I did upgrade my intermediate shaft to a heavy duty (double u-joint) Borgeson and that made a huge difference in slop, but I need to make plans for the ORD bracket and plate kit to firm things up, and possibly look at going crossover steering. Something you'll want to look at on a truck with big meaty tires like that- especially as a DD, you won't enjoy drifting all over the road from steering slop.
No matter what I'd want a real close look under any carpet, and it helps to be able to poke at the underside to see what's under the coating- metal, or rust? Might want to ask more details about the motor- any cam, what carb, fueling system mods, any issues it has had in the past or is having.. since you seem to be a fan of large displacement like myself, you're gonna want to know a lot more detail about what tranny is in there, who rebuilt it, and what they did- is it overbuilt? Stock and just refreshed? A 454, even stock, might be too much for some options. Some other guys here will have better recommendations on what you'd want in there. I'm sure a TH400 is optimal, or the heavier tranny that has overdrive.. 700R4? I forget, sorry!
I'm running a TH350 that is supposedly fresh and overbuilt, and it kicks like a mule when shifting under lots of throttle. It feels rock solid and I like to punch the throttle from low speeds frequently, so at least for now it is holding up fine for road use. I have a feeling when I get around to engine enhancements, it will explode at some point.
If you could get the orange blazer for 7.5k to 8k or so, you'd have some cash left over to source a decent 454, or build one up to your liking for an uprgade project later on!