2X on the pre purchase checklist; Better if you can know the original owner and you get to be the second owner. History is everything, IMO. I bought mine when diesel was over $5 / Gallon, book was $7,000, paid $2600 IIRC. Made my own version of the heath FSD cooler and bought a new Dipaco unit online; $210.
Used ones with "guarantee" are on flea-bay from $69 (edam) or less, if you're a gambler.
I'd stick with the vendors here on this site, as they are tried and true. Replacing all fluids and filters ASAP should be counted into the price of ANY used vehicle. Unless reciepts are provable, assume things may not have been done; I'm not as trusting as some. Cleaning the cooling stack goes with other free or cheap upgrades, if you like doing your own mods. I made my own TM using the existing vac. can, for example, and the stock intake can be modified for better flow. Many variables.
If you're going to tow, these trucks are much more economical than gassers. This is my 3rd Suburban, and wish they had all been diesels.