2017 here coming up on 60k. The 5.3 is probably one of the most reliable motors ever built. GM sticks that thing in everything and there are probably a million of them in junkyards cause the engine will outlast whatever its installed in.
The more I get involved in the car community, the more I find out that the 5.3 is being put into every car imaginable. Besides every pre-LS GM (think GTOs, Square Bodies, G, W & F bodies), they are getting installed in Foxbody mustangs, Nissan 240's, Mazda Miata's, and some nut job is even putting one with a manual trans into a Dodge Caravan.
As stated by others, look for the 6speed trans. Find someone that took care of the car with oil change records. Not that its problematic but if you want to increase the reliability, you can get an ODB adapter that will disable AFM & DOD which will extend longevity even more.