The plumbing under my kitchen sink has all metal parts. The nut which
connects the waste pipe to the J trap/gooseneck has pretty much come
apart and is leaking. Also, the bottom part of the waste pipe looks
really corroded.

Is it advisable to replace just the waste pipe and trap with PVC
plastic parts, or do I have to replace everything from the bottom of
the sink to where it empties into the main drain pipe in the wall with
either all plastic or all metal to assure a good fit?