1. Download latest Ubuntu ISO using a Linux laptop I have rarely used. Check SHA256 on the ISO.
Since you already know about GnuPG, consider checking signature for the ISO file. Ubuntu website provide tutorial about it, see
https://ubuntu.com/tutorials/how-to-verify-ubuntu.
5. Wait for Bitcoin Core to sync to the blockchain.
FYI, current Bitcoin blockchain size is about 507 GB. So sync could take some time, especially if you have slow internet or using HDD.