Unfortunately I think one thing that is going to be working against you is that older Suburbans are starting to acquire a kind of "cult status" in the same way that 4runners and Tacomas have.
It's something I've noticed over the past years and I call it the "internet feeding frenzy." One person on a forum or in some influential web site or YouTube video will say "you know the XXX was a great vehicle" and then that starts getting repeated over and over and over until it starts to become the "conventional wisdom" that XXX vehicle is awesome and desirable, and all of a sudden that POS that you couldn't give away for $1000 5 years ago is being listed on Craigslist with 300k miles and bald tires for $5000 as a "classic."
Then of course, some ************** with more money than brains buys it for that price and tells all his buddies and they go around and try to snap up the remaining ones, again for obviously inflated prices. This is how you end up with 2nd generation 4runners with their hideous "3 point slow" head-gasket-blowing V-6's and paper-thin sheet metal being listed for $6000 or more as a "classic 4runner that will last forever."
Because, you know, it's a TOYOTA, MAN, AND THOSE THINGS LAST FOREVER, MAN!!!"
And of course, once said ************** has spent his $5k on a 25 year old POS, the LAST thing in the world he's going to do is admit that he made a stupid mistake. So he'll pour thousands more $$ into getting it running and then tell lies about how "all it needed was a little maintenance and it was good to go" because he doesn't want to look like a fool in front of his friends or Instagram followers.
My point here is that once a vehicle has "caught on", sellers are going to start jacking their prices up to unrealistic levels. I think that's already happened with the GMT-400 Suburbans, used to be you could get those in running condition for $1000 or $1500, many of them 2500 models.
So what's the solution? IMO the solution is to find the vehicle that has NOT been granted that "cult status" and as a result is largely neglected by the modification/aftermarket crowd and that would be the Expedition. You can still get 1st and 2nd gen Expeditions pretty cheap.