Does the car presently have the proper amount of coolant in it? Loss of coolant can be caused by myriad failed components.
The heater core is gurgling because there's probably air in the system. That, or there's a big enough flow problem that the coolant is boiling, which is highly unlikely because there wouldn't be much of an engine left after a short while of that.
If there are any indications of a blown head gasket, then we've found out why the car was being sold to begin with... It's always a good idea to have a competent shop do a prepurchase inspection before buying one of these cars.
Search heater core, M60 water pump failure, and head gasket failure. If it were my car, I wouldn't drive it until the problem is found and fixed.
best, whit