I wouldn't waste time or money on that bike and just ride it into the ground. Try to take care of it without putting much money into it. The only things I might buy are things I would use on another bike, like a bottle cage or a good tool kit.

A modern bike with a multi speed freewheel is just not worth it. Vintage bikes maybe if they are of quality then you can find some decent freewheels from IRD but that is for those who love vintage bikes. If you are commuting and riding around get something reliable that has modern standards and quality components. We are currently in the 11-12 speed era so generally it is tough to find decent quality below 9 or 10 speeds and as it goes up and up the quality on the lower speeds goes down.