Yes. Go to the Atlassian Marketplace Epic Sum Up page and select the “Try It Free” button in the upper right.
Server and Data Center can only be purchased yearly. Cloud can be purchased monthly or yearly.
Not exactly. You pay the pricing tier you fall into. Example for the server version if you have 25 users you pay the pricing tier for 25 users (11-25 users). But add one more user, which makes your total 26, and you pay the pricing tier for 50 users (26-50 users).
This is Atlassian policy. This means often when you add a few new users you do not pay extra. You only pay when your user count enters into a new pricing tier.
Moving from Server to Data Center?
Learn about Data Center for Epic Sum Up