Head gasket

First of all, I would like to mention I live in my van, and travel cross country in her. So, my odyssey has been acting kind of odd for a minute. The engine light came on, so I had her codes read at a shop. It came back as emissions, and they told me it was an o2 sensor. I continued driving her afterwards, figured it was okay. A few days ago, she randomly would not turn over. I turned her off and waited about ten minutes. She started up fine. A few hours later, same thing. She ran fine afterwards for another day. Then, I noticed her overheating while driving on the highway. Pulled over, and see smoke from under the hood. The coolant reservoir was bubbling, and steaming. We let it cool down, added water to the radiator. Begin driving again, and not even a block later shes overheating again. We then noticed the thick white smoke coming from the tailpipe that smelled like syrup. Whenever I tries to start it again, it ran rough, like it was misfiring. If the van sits idling, it does not overheat. Only when I drive her for more than three or four minutes does she begin getting hot. You can hear the bubbling from inside the car. Help!
We are also homeless and live in the van, so certain things that might be possible to someone with resources, we do not have.