I‘m using debian without all this GUI crap. Should be also fine, shouldn’t it?
Not for me, really. I used Debian for 23 or 24 years, until they decided to to stop giving me the freedom to chose components and go with systemd.
I upgraded from Debian ascii to Devuan ascii and have been using Devuan in servers, desktops, and laptops since then.
If you already like Debian, you'll certainly like Devuan, since it is actually the continuation of Debian but without systemd.
I currently run a prune BTC node and a full LTC node on a Pentium 4 desktop with 6GB RAM and two 512GB HDDs on BTRFS RAID-1 on Devuan chimaera. I got all the hardware for free, and it works very well.
I want to run a full node on this box (I do in others). so I'm planning to replace the HDDs with 1TB ones and rescan the BTC blockchain. :-)
No, nodes don't need to run 24/7. For a while I've been running some nodes only at night and during the weekends, when electricity was cheaper.
What happens if I do some transactions, the switch it off and switch on in a few days?
Your wallet will still have your private keys, ready to sign transactions to expend your utxos. The node will synchronize the blockchain when you turn it back on. If it's been just a few days off, synchronization won't take too long.
What I usually do is having small amounts on LN or on-chain wallets on my phones ready for quick small transactions while my nodes are off.
I'm glad I could help. It's all a little confusing at first, although not complicated.
Perhaps you could start running a testnet Bitcoin node. Synchronization takes much shorter, disk space is much less, and stress for the possibility of making a mistake is also reduced a lot.
Also, it's cool helping run the testnet and you can try any crazy thing you might think about. :-)
I agree with everything except for the Ubuntu part. I rather use a lighter Linux distribution that takes freedom and security seriously, or better yet, use NetBSD or OpenBSD instead.