When I like a reel I want to get as many years  as I can before I have no choice but tp replace it . Usally with reel maker "upgrading" their reels every five years; Shimano Sahara and Daiwa Exceller are good examples, I can't always buy the same reel I had for the last 5-10 years. So , I like to have parts on hand for repairs down the road and/or the odd "accidental" breakages. On a spinning reel what are the most likely parts to fail, break , or wear out ? -Thanks

I feel your pain! I cant get parts for the D series Curados I love so much, anymore. So I bought a few mechnically sound reels that were cosmetically just not 100% for cheap on the board. Now I have my own stockpile of parts to get me through.

The parts more succeptible to wear in a spinning reel are the line roller and the bail spring, get those two and you are practically safe for any problem in the future.

If it's an affordable reel, just backstock a couple of them, or find them on ebay cheap enough to make backstocking expensive ones possible.