Hi John, 

In general, the Toyota Yaris is very reliable, but obviously a 2002 car is getting on now, so you need to expect some wear and tear, regardless of mileage. Potentially expensive things to look out for include failure of the ABS (anti-lock braking system). If the ABS warning light remains on after the car starts, then it could mean expensive trouble. Also, the main engine computer can have issues that manifest themselves in a hesitation under acceleration in the lower gears. If the engine stutters or hesitates it's best to walk away.

But really, most examples of that age will be tatty, but unbreakable. Any other niggles that are common on the Yaris can be fixed inexpensively.