imac late 2015 i7 imac with Monterey through Linux

Have whoever you are getting the computer from do the right thing to prepare the Mac for selling:

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

If it's your intention to buy a used iMac that has Linus installed, I'd be very wary. There's high potential for that Mac to be improperly prepared for transfer to a new owner. A Mac that is not properly prepped can easily become a door stop. It must be released from any previous Apple Account/Apple ID before you can use it fully as your own.

At the very least, the Mac should be prepared following the guidance found here: What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

If done properly, there will be no trace of anything but a fresh install of macOS. If the Mac boots to anything other than the Hello or Welcome or Language selection page, then it may not be prepped.

Run, don’t walk away from this computer. If you are buying a used Mac NEVER EVER buy from:

EBay

Craigslist

Marketplace

Nextdoor

Local PC store

Friend, Neighbor, acquaintance, family

Only purchase from a verified Apple reseller such as Macsales.com or purchase from Apple’s refurbished section on their website.

None of the 2015 iMacs can run anything later than Monterey. Monterey is no longer one of the "most recent three" that vendors like Adobe and Microsoft support.

If you are in the market for an old Mac specifically for the reason of preserving old 32-bit applications (e.g., games) and the vendors have not crippled the ability to reinstall those applications (e.g., by shutting off DRM servers), then buying a 2015 iMac might make sense. (Try to avoid the "price point" version of the 21.5" one. That's the one that did not have a Retina screen, and that often included a CPU that had only two cores.)

If you are in the market for a machine to run current applications, you would do well to skip the 2015 iMacs, and to put your money towards a much newer system, preferably an Apple-Silicon-based one.