Running the Bitcoin core software on an old Linux PC ( I'm using Bodhi at the moment ) is pretty simple, you just need to make sure that you have a large enough disk ( I would suggest 2TB ). You don't need to run core all the time. Once every 3 or 4 days is fine, and then just to sync the blockchain. Of course if you are sending or receiving coins, then you need to run it. The software seems to be pretty robust, and usually recovers if you have an unexpected shut down. Of course, you shouldn't rely on this, and close core and wait for the "don't turn off message" to clear before shutting down the computer.