@Ron Glad that was useful. I’ve had a couple more thoughts over the past couple of days: while apps that use the OS crypto and HTTP libraries (which is most of them that do any connecting to the internet) won’t be able to access servers that require TLS 1.2, 1) most apps probably don’t need to talk to servers that require TLS 1.2 or higher secure connections, and 2) it looks like Firefox enabled TLS 1.2 by default in Firefox 24, and Firefox 48 and Firefox ESR 45 support Mac OS X 10.7, so if you happen to run into a web server that requires TLS 1.2*, you can still visit it in that old Firefox (or possibly an old version of Chrome; either way, between SSL and JavaScript, the old version of Safari in 10.7 is going to struggle with much of the 2019 web, so you’ll want Firefox or Chrome). Also, the 4 GB RAM limit is going to be a bit tight, but should still be useable if you aren’t doing heavy browsing. So hopefully that’s more glimmers of hope :-)
The second bit is that it’s much easier to stay frozen in time than bring a new-found old Mac back to life decades later ;-) If you had been using those Minis in the late 2000s, you would have already had installed lots of software, which would have kept working as-is (you wouldn’t have been able to upgrade to new versions and get new features), but you wouldn’t have had to chase down old versions of software that would work on the old Macs/OS versions. (Thankfully, many indie Mac devs like Red Sweater and Bare Bones and Flying Meat are good about keeping those versions available, if unsupported, for people with old Macs.) You’ve taken on a minor challenge, but it’s definitely do-able with defined usage goals. Feel free to holler if you need more advice once tax season is over and you dive in :-)
* I haven’t found lots of sites that require TLS 1.2, but I’ve run into enough of them to find it annoying on my very old MacBook Pro.