What needs to be replaced/repaired on Avalon with heavy smoke and knocking??

What is your question?

I have had the car for almost two years. My dad bought it for me from a family that had taken good care of it- or so I thought. It is salvage title, but there isn't any obvious damage. After 2 weeks, it began to put out very thick, white/blue exhaust. I was not sure of the problem, and then I saw the oil light on. I kept filling with oil so the car did not over-heat as a mechanic suggested. I then took it to a mechanic that told me that the headers needed to be replaced. I spent 1900$ with this mechanic but he did not replace my knock sensors. He instructed me to buy the knock sensors and he would put them in or free. After a week, the check engine light came on again and it began to smoke heavily. I would replace the oil multiple times every week. The codes always come up as a misfire on cylinders 1 & 6 and then the knock sensors as well. I hope to get it fixed asap, but I am a student and also a heavy commuter. Any help is appreciated.