2006 Toyota Sienna overheating and now knocking?

This is my first post so bear with me, please. About a month ago I was given a 2006 Toyota Sienna. The first two weeks it was driving like a dream and very quiet. It has about 350,000 miles. The engine is a VVT-i V6 3.3. All of a sudden it would start randomly overheating. To be clear it was never driven but immediately pulled over and shut off. The overheating was not all the time. I started by doing the simple changes such as a new thermostat, new cap, new hoses, new belts, and a new heat sensor. When done I flushed the radiator. Some dirt came out and the pressure was pretty forceful coming out. The whole time I couldn’t find any leaks before or after the changes. It drove amazing again with zero problems for about a week. One day driving the temp shot up smoke started rolling and oil light came on. The van shut off and I was able to coast it to a safe area. After letting it set, I checked oil and it was almost non-existent. Two days prior I had checked the fluids and everything was good. I had put oil in and filled the antifreeze. Since then I have barely driven it but I am a mom with no other transportation so I still had to run a few miles for necessities. The van now overheats but never shows hot on the gage. The oil light stays on regardless. I do make sure that there is water at all times. A knocking has started a couple days ago. Does not happen at startup. Just happens when I accelerate. Once I get going the knocking stops. Its only at lower RPM. I had it looked at yesterday and was told the radiator was very bad but then told the engine is too. That I need a new engine. He never really said specifics on why the engine is bad. I can’t afford to replace an engine and can’t afford a new vehicle. I do all my own work unless it’s something major then my friend helps. Can anyone please give me ideas or where I should start to look. Please.