Can a 2015 iMac Model A1418 with Catalina run Big Sur?

NEVER EVER buy from Ebay. If you only have $600 please continue saving until you can afford something. For about $1100 you can get an Apple Refurbished Mac Mini that will far outperform a 2015 iMac. You can also finance it interest free for 1 year too.

That price would also include a budget for a $100 display which you can easily find brand new.

Refurbished Mac mini Apple M1 Chip with 8‑Core CPU and 8‑Core GPU - Apple

IF you are buying a used computer, PLEASE do yourself a favor and never ever buy a used computer from a stranger. If you cannot afford a new one, then buy from a reputable source that will stand behind their sales. I would recommend you visit macsales.com or consider a refurbished Mac directly from Apple. Go to Apple.com then navigate to the bottom of the page and click the Refurbished section.

Apple refurbished products are identical to new, have the same warranty as a new, are eligible for AppleCare, can run current (and future) versions of Mac OS and can save hundreds of $. This is MUCH MUCH smarter than buying used.

These forums are full of posts from people that have purchased a used Mac and have only inherited a disaster. The only thing I'm trying to get you to see, is if you need to buy used, do it right!!!

Yes.

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Your welcome and good luck.