For a while now my previa would shake when cold. It would shake at a stop but it would run fine. The shaking would be minimized once the van ran for a few miles. So a few days ago she was running fine. So I was at a red light and she started shaking violently, threw out a lot of white smoke from the exhaust and then just died. So I was hoping someone here knows what the problem could be and how much repairs would cost. I just want to see if it's worth fixing. A friend of mine said that it could've blown the head gasket.

Violent shaking and white smoke from the exhaust is most likely going to be diagnosed as a blown head gasket.
On that model, with difficult access to the engine, the estimates will reflect that. It will be expensive; perhaps a couple thousand, plus or minus. And that won't include any damage that might be found during disassembly.
Have this properly diagnosed and get a written estimate. Then get back here to let us know before you let go your hard earned money.

Sorry to say but I think it's time to give it the sign of the cross and it's last rights.
Sounds definitely to me it popped it's cork. (Headgasket)