For eventing, look for SOUND horses who have absolutely no health problems(quite obvious)
The conditioning it takes to create a good eventer is ridiculous. Also, look for horses that are generally sound without being shod. Of course shoes help with the different/harder terrains, but you don't want to spend your days ripping your hair out because you can't keep the horse sound!
That's why my boy is no longer an eventer...his previous owner had such a hard time keeping him sound that it just was no longer worth it.