Strike Windows, Apple and other non-
free Operating Systems off your list.
Especially since you're willing to run the full node through Tor, moving from Windows or Apple to Linux is an awesome move towards privacy. From all the distros I've tried, I'd personally go for Debian. Feels great, looks nice and I consider it to be quite newbie-friendly. I consider that using Tor makes no sense if you're using an Operating System filled up with bloatware and spying stuff. You either move to the more privacy-oriented zone or you don't at all (personal opinion tho).
As @ranochigo said above, the rest depends on what your purpose of running a full node is. I'm way more paranoid about security and surveillance than the average Joe, so this is what I would do if I were you: I'd get a cheap PC (I'd personally get at least 4GB RAM tho) with sufficient disk space (at least 500GB) to run a full (non-pruned) Bitcoin node through Tor and use it
solely for cryptocurrency purposes. Having a PC dedicated for crypto activity lowers the chances of getting a virus to do malicious stuff, a virus that you'd get from regular browsing and activities on your PC.
You'll want to get a disk of at least 1TB if you want to run a node without pruning. Pruning will discard the unnecessary files that you won't need if you don't care about the things mentioned.
Why 1TB though, I wonder? Like, isn't the current blockchain size around (or less than) 350GB? Is that just to avoid spending yet another sum of money on more storage on the long run?