The heater cores blow because of the low quality radiator caps VW used on this generation of cars. When the heater core in my MK2 failed, I took the car to a specialist who had a special jig on his shop's wall just for testing the radiator caps to make sure their valves opened at the right pressure. He'd get the caps in bulk from Germany and test them before installing them. Many of them were defective before ever being used. This was at least six years since they started using the destructive radiator caps, and VW still hadn't resolved the issue.
Another issue to look out for in these VWs is ones that have been driven by BMW drivers. BMW shifters were designed to provide strong resistance to being shifted into reverse. VW shifters were designed to be pushed down on when selecting reverse. There was a tiny thumb concealed under the shift boot that needed to go into a track to select reverse. When a BMW driver selected reverse in a VW, they overcame the resistance provided by the thumb, it snapped off, and then there were no defined gates for reverse, first or second gears.