I'm fixing up an early mountain bike with all original suntour parts and making it into a commuter. I need recommendations for replacing the existing 6-speed freewheel and rear derailleur.

The thumb shifters work great, as does the front derailleur. The rear derailleur however makes a ton of noise in most gears, and it seems to be crooked. I think the noise is the chain hitting the side of the derailleur cage as it enters the pulley. On top of this, the jockey wheel is looking near end of life (pointy teeth) and the two of the six cogs on the freewheel are missing teeth.

So I am wondering if I should replace these two parts and keep the shifters, etc., or ditch the setup for something else. I briefly though about switching to an internal hub, but I'd rather not put too much into this project. What would be a solid but inexpensive replacement for these parts?