Hey guys - I had my car fixed by BMW dealership afterall and here's what happened - it may help somebody else with similar symptoms as mine.
Before replacing the thermostat, I decided to try air-bleeding my system first. Jacked the car, put the heat on highest and ran the cabin fan on slowest all while adding the coolant and checking the bleeding screw. It some 20-30 min. Tried starting the car - and found drained battery. Called CAA and they said that battery is dead and needs to be replaced. Although skeptical about CAA diagnosis, at that point I had the car towed ('flatbed-ed') to local BMW dealership. Final word - replaced battery and overflow bottle sensor and bleeding screw. Got my car back next day. Knowing this, replacing the overflow bottle set DIY would fix the problem.
Thanks everyone for input and help!