Hi!
Depends on your forum’s size and activity.
If it’s small, a CX22 for 5€/month will be fine.
A CAX11 (ARM architecture) has the same specs and price, but I find that Discourse runs even better on ARM than x86, not sure why. The drawback is that there’s no official ARM-compatible #email-receiver image, if you intend to use it. There’s a fork that works instead.
Yes, you can’t install Discourse without a domain name.