While the older Raspberry versions (A/B/B+) may not be up for the task, you can easily use the Raspberry 2 for a Minecraft server. At least 5 concurrent users are no problem at all, depending on your bandwidth. My Raspberry 2 did not use more than 300% CPU in a test, which is actually 75% – most of the time it was even below 200% (=50% on the Raspberry 2). It seems that Minecraft scales well over the 4 cores, at least with the version i am using.
You already know how to run scripts periodically with a cron job if you read my last blogpost about the Raspberry Pi. This time i will explain how to run scripts on startup. Because you do want to restart your Minecraft server when the Raspberry Pi crashes, right? 😉