The main places to chech for rust are, under the carpet, under the car especially the frame rails, the little "shelf" above the tail lights and under the hatch, check the cowl panel, around wheel wells, front fenders between the door and front tires. Pretty much everywhere else too.
A car in primer can mean they are trying to cover something up, it seems strange that someone would go to the trouble to primer a car and not paint it.
Ask lots of questions sometimes that is the best way to get to the truth. Makes sure everything they tell you "fits"
Good luck