Welcome to BF.

I absolutely would not put any money into that bike.

If it works for the time being, keep riding it until you find something better to buy on Criag's List or ebay. You can often get a good used bike from the 80s for around $100-200. Then sell that one for $40 and enjoy your new ride.

Where are you located? Maybe someone can set you in the right direction for a good shop or place for used bikes.

It also might be worth it to put a Want to Buy ad on your local Craig's List with the size and price range you are looking for. You never know what might pop up.