At that price point, something used and not particularly recent. Even an old laptop will be good performance-wise, but not necessarily storage-wise.

However, buying a laptop to run at home might not be the most reliable or economical way to host Lemmy: you can rent a virtual server for not much, provided that you have access to a computer to remotely access the server with.

I have zero experience with YunoHost, so I don’t know how that may or may not limit the options. Probably the most common way is to use Lemmy-Ansible. It’s easy (well, relatively speaking) and is maintained by the actual lemmy devs themselves. Managed services (ones where you don’t have to deal with the server) should also exist for lemmy, but I’ve never used them.