Bone stock 95, detonates, help please?

[ChesteR]

This is kind of a tough situation, but I am about to purchase this 95 GT and it detonates under load above 3500, Is this usually an easy fix? I just put some 92 octane in it on the test drive, because it was empty, how long would better gas take to reduce/rid the detonation? A couple other things, it runs a bit warm (which may be a result of the detonation)and the A/C doesn't work, is there something in the A/c system that is more prone to failure than other parts? I am having the car looked over by a ford dealership tomorrow, but I wanted an indication of what the problems might be beforehand. 66K, 5 speed. Thanks much