Is this worth buying: iMac 21.5" (late 2009 model)

Whether this iMac is worth it or not is entirely your decision.

That iMac tops out at macOS 10.13 and won’t run more current software, so you won’t have access to a variety of apps from Apple (various of which now require Big Sur), among other vendors.

That’ll be four macOS versions back later this year with the upcoming release of Monterey, which usually means support for Microsoft apps will be ending then (as they and some other app vendors tend to support the three most recent macOS versions), too.

That model is difficult to upgrade memory, and it’s probably also a hard disk drive that’s now a ~dozen years old, and should that fail you’re limited to booting and running from external storage.

You’ll also want to budget for an external hard disk for Time Machine backups, whichever Mac you choose.

Welcome to the Apple Support Communities.

That is an old, and probably slow computer, that can only run up to macOS High Sierra 10.13. It’s up to you.

Perhaps you should look at refurbished Apple products here: Certified Refurbished Products - Apple.

Also, if you want a used Mac, do NOT get one from eBay, Craigslist, etc. Get it from a reputable seller, such as OWC.

Jack

Probably not.

A Late 2009 isn't worth much more than the shipping cost, and even then only for legacy/retro use or as a low-end *nix machine.

I wouldn't recommend one for student use. It is also much too old and slow if you were expecting to run current versions of XCode or other Apple exclusive design/development/editing software.

Rather than considering a 13 year old computer, if you are 18 or older perhaps it is time to work towards establishing some credit and buying a newer Mac. If you are a child then keep saving until you can afford a newer Mac. A 2009 21.5" has zero value at this point.

No, unfortunately, unless it’s free and comes with owner-provided support.