The coolant additive you use isn't massively important but keep in mind that you get what you pay for. What is really important is that you use a proper corrosion inhibitor and in a suitable proportion for your region.
If you have to use a heavy mix of antifreeze the you may wish to consider using water wetter to bring the cooling efficiency back up.
I would say that previously your heater was airlocked and/or partially blocked. Draining the system has cleared that problem and highlighted the fact that the heater has a hole in it. In terms of a quick fix the only thing you can do is close the heater valve or better still bypass the heater in the engine bay.
If you want a heater in the car then you will (at some point) need to take a look at the radiator behind the dashboard. They aren't hugely difficult to take out but equally they aren't easy either.