As Gareth has said, if you already have a monitor and peripherals, just grab a Pi, power supply and a small SD card and you're good to go. You can tape the Pi to the back of the monitor for an ultra clean setup or get a smart looking case and make a feature of it. Either way, much cheapness and if you get to the point where your Dad doesn't need it, repurpose it into a PiHole adblocker, a RetroPie gaming system, a NAS or a million other things. So much you can do with a basic Pi. A 3B+ would more than suffice if they're on sale but generally the 4's are the same price as the 3's in most places so may as well get the newer spec.