Q: The car I'm interested in buying has an engine knock

Engine knock by definition means the combustion taking place is mistimed or improperly detonated. More technically it means the explosion(s) has taken place partially or completely outside of the "envelope" (or area in the cylinder that is designed to push the piston down correctly). This will create an audible knock that can be heard during idle and while driving. One of the reasons it happens is due to the wrong octane rating, however in your vehicle that should not be an issue.

The spark plugs/wires and carbon deposits can also cause engine knock. You may want to inspect and possibly replace the distributor, spark plugs, or wires. Inspect the plugs for excess carbon build-up, this will tell you if the carbon deposits are a factor.

There are other parts that can cause a knocking sound that is not by definition "engine knock". If your engine’s rod bearings are wearing down, they can cause a knocking and be a sign of a much larger issue. You will want to look further into the knocking and see if you can pinpoint where it is coming from.

I would highly suggest having a pre-purchase inspection be done on the car by a certified technician, like one from YourMechanic. By doing so they will be able to diagnose this issue and pinpoint any other issues that are present prior to buying the vehicle.