Wiping and reinstalling OS and apps on an old 27" iMac?

FYI, current versions of Pages, Numbers, and Keynote require Sonoma or later. For 27" iMacs, that means that only the ones released in 2019 and 2020 would be able to download these applications "from scratch". Apple once sold iWork on optical discs, so for a Mac running a really old version of Mac OS X, scrounging up a used original copy of iWork from some place like eBay might be an option for a new owner.

Be aware that unless that iMac can be upgraded at least as far as Catalina, a new owner may have difficulty finding much usable software for it. If that Mac is old enough that you can only upgrade it as far as High Sierra, it might be a great machine for running old 32-bit games that won't work on current Macs and versions of macOS – but it may not be the best idea to give the machine to someone who does not know exactly what they're getting into.

The current version of LibreOffice will run on Catalina, although when the first 26.* ("2026") release comes out, the requirement will change to Big Sur or later. There is a download archive of older, unsupported versions that will run on older operating systems, and a documentation/system requirements page with links to them is here.

With regards to Web browsers, Firefox is probably the best at supporting old versions of macOS. Its current version will run on Catalina and later. An older ESR version that runs on Sierra, High Sierra, and Mojave is still eligible to get "critical security updates" from Mozilla, although they may stop providing those soon. Most other major third-party browsers require Monterey or later.

If the Mac is working today, do yourself a big favor and create a bootable USB installer for macOS.

The year that iMac was released will determine which is the last macOS version it can run, and that is the OS you should create a bootable USB flash drive for.

Find the year your Mac was released by clicking Apple menu  > About This Mac.

Do not erase your Mac before you do this. That is just asking for problems and headache, especially with the oldest iMacs.

Create a bootable installer for macOS - Apple Support

With a bootable installer in hand, wiping and reinstalling macOS becomes a fairly easy task, realistically.

If that Mac shipped new with Pages, Numbers, or Keynote pre-installed, or if it was purchased from a third-party that installed these applications, you cannot get them now. Only the currently available version of these applications are available from the Mac App Store with v14.4 requiring macOS Sonoma, and once Apple Creator suite is released, the updated versions of these applications will require Sequoia.

Anything you install on the reinstalled macOS that comes from the Mac App Store will be tied to your Apple ID, and that will complicate things for anyone you gift this Mac too, provided there are updates available.

If you install a version of macOS later than macOS Mojave 10.14.6, the iWork '09 and earlier suites are incompatible because they are 32-bit.

it's not that difficult at all. please visit What to do before you sell, give away, or trade in your Mac - Apple Support to return the Mac to an "out of the box" state.

Here's the Apple document covering preparing for selling, giving away or recycle: What to do before you sell, give away, or trade in your Mac - Apple Support.

If it has an OS installed capable of running the current software versions, those can be reinstalled from the App Store. Otherwise, the new owner will need to have access to an Apple Account previously used to download them and the developer will need to have left a compatible version of them available.

For software which didn't originally come from the App Store, it depends on what the publisher's website offers.

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