Forgot a biggie: most relatively recent road (<20 years old) bikes I’ve inspected that have brifters (integrated brake-shift levers)…. the shifters are pooched. About half of these levers are recoverable via. rebuilding or flushing them out with WD-40. The other half require a right shifter replacement, because that is where the shifting action occurs. So factor this cost and repair time into your used bike economics.
I ride Campagnolo Ergopower shifters, and these can be restored to full functionality with $25 worth of parts and an hour of picky mechanical work. Shimano STI shifters – I’ve about a 25% success ratio in bringing these back to life.
BTW: many used bikes with brifters have been crashed. When you unwrap the bar tape, you realized that the brifter bodies are cracked, and so are unsafe to ride. Put this into the show-stopper category.