Some things I wish I knew to look at before I bought my 04 Denali with no known service records.
Exhaust studs constantly break, can be solved with Kral clamps. However, the permanent solution is always replacement.
Oil pan gaskets are issues, replace pickup tube oring at the same time as gasket. It seems like oil leaks are a general nuisance issue, at least with this generation.
I haven't had a problem with my trans, but the impression I get is the transmission is more like to have a catastrophic failure than the engine.
Check to see if drive cycle is ready, a decent single directional scanner will tell you. If is not, ask why was the computer reset recently. People may clear codes right away to hide the fact it is having throttle issues on drive by wire vehicles. (2003+)
Of course do your typical used car buying inspection, fluids, brakes, suspension, systems functionality, suspension noises, general drivability.