1. The car shape/technical condition is the most important thing. Starting from the bottom, underneath (corrosion), not from the top (dents and scratches on the paint - like most people "check" the cars). Body, suspension, brakes, engine, electrical things, exhaust rust, fluid leaks, noises.
2. Mileage itself not so much (unless there is LPG - danger of burnt valves, because of neglected maintenance, like valve clearance adjustment, almost no one does that in 8V engines). So LPG is a biggest red flag (right after body/suspension rust and black thick deposits inside the engine).
3. Service books can be "forged" (you get fake one, so you think you have "perfect" car). Personally, I prefer stack of invoices/receipts (for car parts and services) from previous owner when I buy a car. I do the same (all repairs/purchased parts are documented). I know, used car dealers don't do this.
4. Official service intervals are questionable, especially oil changes ("long-life"). So even "car serviced regularly in the official dealership" is not a good thing too (not always). Watch out.
5. You are buying used car. You will pay extra anyway, more or less, soon or later. There is always "something" to do with "new" car, some service (oil, brakes etc.). Don't expect there will be no additional costs after purchase (because "car was serviced" and it's "documented").