A crunching sound could be any number of things, so it's worth investigating systematically before pre-emptively replacing anything. I would only divide the parts into two categories for now: Works and Broke. The third category, "would be better if upgraded" could be put off for the future.
For the crunching, I would try to investigate precisely where it's coming from if possible. If you loosen the rear wheel and drop the chain, then you can spin the crank on its own. It should sound smooth and not have any play. If it has play, the bottom bracket might benefit from cleaning and re-adjustment, which requires a small handful of specialized tools. Park Tool website is a good resource for videos on how to do the basic operations. If upon trying to clean and adjust the bottom bracket, you discover that it's worn out -- pitted bearing surfaces -- then that's the time to just replace it, so you can reassemble and forget about it for a long time.
Likewise spin the pedals. Do they spin freely with no play?
Are the cranks tight, or can you wiggle them around? Is the chainring tightly attached?
Now that the chain is off, you can independently check the condition of the rear hub in the same way. And the front wheel, and the steering column.
Piece by piece. If you leave everything a bit cleaner and better adjusted than you found it, you'll have a better bike and perhaps one that will last a long time.