Mixed Tire Types

If the owner installed a mix of tires like that, I'd say he didn't treat the tires very well. The tires could have been well-abused and ready to fail at any time. May *look* okay but you won't know how often he towed over 65 mph and/or under-inflated, hit numerous potholes and curbs, etc.

I would install a set of new for your own safety and prevention of damage.