Buying second hand vehicles always involves a fair element of chance. The rest depends on how well you can scrutinize the product before committing, thus reducing risks of breakdowns, repairs and repentance.
Dependability, of used vehicles are largely influenced by wear and tear inflicted by previous owners' driving habits and the frequency of maintenance.
If comfort is a high priority, then a saloon shaped vehicle like an Astra, Esteem, Cielo etc. is recommended. Hatch backs will be a compromise on comfort.
The maintenance cost of the car will depend on how much you run your vehicle. An old diesel vehicle will have higher maintenance cost although operational cost will be lower.