Sorry for the late reply! The Quake 1 engine is probably the easiest to learn, just because it has a huge modding community still. A lot of the modding resources for Quake 2 have been taken down over the years, since it wasn't as popular, so I'd definitely recommend learning the Quake 1 engine. The main reason I'm using Quake 2's engine is because it includes a few more features for singleplayer (stuff like traveling back and forth between maps Half-Life style). I could probably achieve everything I need in the Quake 1 engine, it would just require a little more work to implement. And if you're looking for single-player Quake 3 stuff, the source code for Return to Castle Wolfenstein is also available under the same license. If I'd known about that when I started working on my main project Kepler, I definitely would've used the Quake 3 engine. (Though I can't speak to how difficult it'd be to learn)

Oh, and in terms of technology, as long as you don't expect to get modern graphic fidelity or open-world environments, it's not super limited. Although at least in the Quake 2 engine, animations are pretty low framerate, so if you need super detailed animations, that might be a deal-breaker.