Question

I just bought a 2000 740 il 3 mnts ago. The coolant light keeps coming on as though coolant needs checking. However, when checking coolant it always checks full. I have to put oil in every 3 to 4 weeks. I did have a severe problem when cranking a puff of white smoke would come out and stop. Did not run hot. So i started using Prem-gas (was using mid-grade). The problem seemed to resolve itself with the white puff of smoke, very little would it happen anymore. Now 3 months into owning, tried to run heat (after having running the air since purchase date) it ran hot. Checked coolant, it was full. Checked oil, yes did need to add 4 qrts. Still running hot with all full in oil and coolant. Whats happened? Thermostat? Pump-doubt because no sign of leakage? Head gaskets-hope not? Help me please!!!!!