The rule of thumb for me is that I observe the roads for a while and see which cars do i see the most. also which cars are in taxis, taxi drivers tend to buy the most reliable and the cheapest ones so they can save money. if you buy the same model in private. you can be sure there will be plenty of shops, parts and easy fixes for that car.
I have tried the same in two very different countries and it was worked like a charm for me so far