One small detail the article forgets to mention, you will still need at least a 256GB card and pretty soon a 512Gb one to run the full node, the package doesn't come with one and the cheapest is around 30-40$.
Funny how the site they mention
http://www.raspberrypifullnode.com/ still talks about 64 gb

USB hard disk? Especially a powered dock, 2tb used is only around $20. Plenty of space for the blockchain.
Make sure it's low power. The new Pi is more power hungry.