I've had buyers remorse after almost every old thing I've bought. I always feel like I paid too much, and this is after I looked it over thoroughly. I overpaid for my 65 Mustang, because it was a "coupe", but dang it, it was legit rust free. I definitely overpaid for my 68 firebird. It ran crappy when I bought it (OHC 6 car) and I thought the guy selling it was a dope, so I figured I could straighten it out. Turned out the head was rebuilt wrong and it was only building full compression in 3 cylinders. The other 3 were half dead. What do you do at that point? Insist on a compression test? Again, another car I bought because of how legitimately rust free it is, my long term plan was a 400 clone anyway, but I wanted to drive it a bit, you know? But the cost of rebuilding the rare OHC 6 - and just finding someone to do it, is Pontiac 400 money...
I've looked at your for sale ad. I have to agree with Fine 69, as far as Blazers go, it appears to be "solid" overall. Rust free.... no. Solid, yea.
I've read you want to do a repaint and an LS swap anyway? This blazer appears to be original, as in no boogered up previous repairs, just some bondo over what appears to be pretty minor rust. This is a game of devils advocate, would you rather have this blazer to rebuild or buy another one that appears to have been done right, only to find there's welded in panels everywhere, or all aftermarket metal?
I'd inspect the piss out of everything you have. Remove door panels and everything with access and look inside all of the panels. Look for rust, dents, and holes covered up with bondo. If it appears to be mostly cosmetic stuff, I'd build the blazer you have, because the next one could appear to be better, only to find worse.
I like the avatar pic... I get it... you're upside down on the blazer 
I was wondering if anyone would get that?