Ask a Ford Dealer to run the car's VIN service history. That should tell you whether the HG was replaced (by a Ford Dealer) or not and when. If the HG hasn't been replaced (it affected all 2016 RSs); I'd walk away and keep looking.
Speaking of service history; was the car getting regular services at the dealer, or is there documentation the owner was doing it themselves?
As with any used vehicle; general mechanical and electrical state; e.g. does EVERYTHING work, or are somethings worn and will they need to be replaced?
Depending on your personal preferences and prejudices (what's acceptable to you); make sure you know what state of modification (if any) the car is in. You don't want to get stuck with somebody's partially completed project car, or one that's got some half assed mods that need to be redone/fixed, and end up with thousands in post purchase costs to "get it right".
Good luck!