considering buying a 2018 used mac mini to store my pics on.

There is no reason to get and use another Mac for your Pictures.

A better option is to get an external HDD or SSD to store your Pictures on.

see > Move your Photos library to save space on your Mac - Apple Support

At the present time, the 2018 Mac mini is capable of running macOS Sequoia, the most recent available.

MacOS upgrades have been free for over 15 years, or so. There's no reason to expect that to change.

If you already have a Mac, there really is no reason to buy another for the storage of your pics. An external drive can be used for that at much less cost.

Between us, depending on the specs and price paid, if you only aim to store pics, the 2018 may be overkill. This is a very capable machine and as said before, up to now, it supports Sequoia. You would probably never have to pay for a macOS update but eventually it could be limited for upcoming upgrades, we still don't know.

I use the 2018 as my main computer to do audio and video work and much more. It is loaded with a 2TB internal SSD, 32GB ram, and has the advantage of being equipped with 4 TB/usb-C ports (while newer mac minis have only two if I'm not wrong, plus two USB3 ports). I run it with two monitors (and have done 3 in the past).

A 2018 Mac mini can run anything from macOS 10.14 (Mojave) to macOS 15 (Sequoia). Vendors such as Microsoft and Adobe have a policy of supporting the "most recent three" – currently (Ventura, Sonoma, Sequoia).

How to download and install macOS - Apple Support

Although Apple has not made any official announcements, it seems likely that Sequoia will be the final update for the 2018 Mac mini. The oldest Mac compatible with Sequoia is the 2017 iMac Pro – a workstation that uses Intel Xeon processors. When Apple moved from PowerPC to Intel, there were only two versions of Mac OS X (Tiger & Leopard) that ran on both CPUs. With the Intel to Apple Silicon change, Apple is already up to five.

To add, there's no telling if the 2018 Mac mini will qualify to run the next major upgrade of macOS that is likely expected late next year, or any others beyond that. Only Apple knows.

If you’re still deciding…

Other options for self-hosting of photos include NextCloud with Memories, and Synology NAS boxes can host photos, as can Piwigo, and LibrePhotos, among other apps.

The Synology NAS boxes can also provide Time Machine Server, among other options.