Ford Focus - Blown head gasket?

About 10 days ago, my 2003 Ford Focus LX died on me. It was diagnosed as a bad battery and a leaking radiator hose. Both were replaced 8 days ago. One day after the repair, my check engine light came on and the local Auto Zone diagnosed a misfiring cylinder, which they attributed to a bad spark plug and stated that I could drive it safely without issue until I could afford to replace the plug(s). The car was idling poorly (rumbling) and blew white smoke when first started, both problems were gone after the car was warmed up. I drove it for another 6 days and on my way to work yesterday noticed that the fan was running when I didn't think it should be (highway driving, had come to a stop and heard it going). All of a sudden upon accelerating from that stop, my temp. gauge shot up into the red. I stopped the car, pulled over and opened the hood. The coolant level was low (which seemed unusual considering the mechanic had changed the radiator hose just one week before and says he refilled the coolant afterwards). I let it cool for about 20 minutes then drove back home about a mile away. I ran the heater to help cool the engine a little (it was still near the red zone on the gauge) and after 30 seconds, the heat wasn't even hot anymore although the engine still was. At home, I let the engine cool down for about 40 minutes with the hood up and upon removing the coolant reservoir cap, there was a large amount of pressure released (more than you'd expect). I refilled the coolant and drove it 3 blocks up the road to my local mechanic who called me later to give me the sad diagnosis of a blown head gasket, which he said would cost me about $2k. After searching online, I believe it could be true but want another opinion and also wonder if it didn't have anything to do with the initial radiator hose repair by a different mechanic (maybe they forgot to refill the coolant?). The car has 100k miles. I am also thinking about using Thermagasket or some other similar product, just to get me by long enough to save for a new car (the mechanic has offered me $650 for the car as is). What do you think?