I also think buying a Pixel phone and running the web installer is the most secure option for beginners, as you don't need to trust any broker. But if you feel like buying it with GOS pre-installed, you can always verify the key hash on boot so you know it's has not been tampered: https://grapheneos.org/install/web#verified-boot-key-hash
treequell Buying the phone from Google is recommended, but buying from most stores new will be fine.
Hard disagree for privacy. You'd have to give Google some payment and address data which might allow them to connect you to the device. If you want to be really sure it is not traced, buy it at a store (preferably far away from your home town, e.g. on a holiday journey), wear a mask (thanks to Covid not really suspicious) and pay in cash. Or have somebody else buy it for you in cash.
Don't buy a used device because you don't know what usage history is attached to the hardware identifiers and this might raise unwanted attention.
Since you probably buy a Pixel 6a or later models, you can expect 4-5 years of GOS updates. If you want to use the device for that long, it's better to take a few extra steps to mitigate as many security and privacy risks as possible. Of course these are just my two cents, do whatever fits your threat model, it's great already that you'll be on GOS.