Build quality to me is the quality of the plastics, the closeness of shutlines in the bodywork, the smoothness of the engine, and many other points.
Take a cheap brand, it has nasty rattley plastic on the dashboard, large gaps between the doors and bodywork, rough engine and develops rattles and squeaks before it's a few years old.
Sit in a 1990's BMW on the otherhand and the shutlines are narrow, the engine is still smooth (even the 4 cylinder) and the dashboard plastics are still black (not grey!).
I would opt for a decent built older car rather than a new car everytime.