If AC is your biggest worry and you're about to sell out anyways.... Get some soapy water in a spray bottle, then shoot some compressed air into the system. If you use a manifold/gauges/charging hose it'll make life easier. Maybe you'll get off easy and find a giant leak somewhere visible.
If ya find nothing then... the system is already open and you're selling out anyways. And... if you find/fix the leak on the cheap, toss in a new rec/dryer, get a HF vac pump, then try to find some R12. I hear Freeze 12 also works, but it's definitely not the same thing