The hardware that you should look at for running hope assistant depends greatly on your plans and requirements
https://www.home-assistant.io/green/
This is a great product if you are just starting out and want to learn. It allows you to jump in without alot of complicated setup.
It is however limited as it is a very low poer and low performence hardwaree. If you are looking to run alot of addons (an addon is not an integration) to run addition services then you will find the green will get bogged down