At what mileage point is a car or suv not worth purchasing
That all depends on make and model of the vehicle. Some of the larger SUV's can run a long time if taken care of. So find a make and model you like, check the reviews and problems people have had with them and if your still wanting it then have it checked over by an independent shop before committing yourself to buying it.
I would normally only buy a used car if it has well under 100,000 miles.