Fan problem will be the resistor pack. Common issue. Google "Commodore fan speed resistor" and you'll find a few forum sites have threads with pics showing how to replace it.

The heat issue could be one of 2 things.

1. Heater valve in the engine bay - this is a vacuum operated switch that diverts the water away from the core when the temp dial is on 12 o'clock or colder. When you turn it to heat, a vacuum switch on the case, near the core's pipes, opens up, triggering the heater tap in the engine bay to open & flow hot coolant into the core.
Check the vacuum system is working OK, and the tap hasn't corroded it's valve flap.

2. vacuum operated flaps - these divert the airflow through the core.
It could be that they are not fully opening & closing - lack of vacuum at the actuator, or something physically jamming them could be the cause.
Also, the seals on these flaps can deteriorate over time & let air bleed past, and via the heater core. Unfortunately you have to pull the whole box out to fix these seals.
I've found the heater box seals, along with VN-VS headlining foam, seems to fail so much sooner in cars that have been smoked in - must be something in the smoke that attacks the polymer.