Arch. I need the AUR for certain applications, and the high degree of customizability and opportunity for learning appeal to me as a relatively new-ish Linux user (going on a few years now, most of that time having been on Arch).

For system files/configuration on my machines, timeshift set to run once a week.

For family photos and shared files, I built a pair of SFTP servers made from old HP thin-client PCs at two different geographic locations which automatically sync to each other once a day via cron job using vsftpd and lftp. Each one has both an NVMe and SATA SSD which run in a software RAID 1 configuration.

For any other files, a second local server also using vsftpd and two SSDs in USB enclosures. I manually back them up using rsync on an irregular basis.

I would say a used Dell or HP business laptop would be a safe bet. Most business laptops have decent keyboards, replacement batteries will be relatively easy to find, and user-serviceable RAM is the norm. Given the not especially high processing power needs, probably the middle-specced ones with a few gens-old i5 will be dirt cheap and work fine for your needs.