Go for a Raspberry Pi. It’s what most people use for a number of good reasons, and in particular any beginner should start with a proven solution so he can focus on learning openHAB instead of having to mess with the server. The less mainstream your server the more work for you.
Use openHABian to get started fast.
It’ll also keep beginners from making a number of bad decisions that can lead to frustration quite quickly.
This is useless advice. See Corrupt FileSystems every 2-3 month? - #20 by mstormi.
It is true, however, that anyone to use a Pi (or other SBC) needs to take precaution to not get hit by SD card corruption or HW outage in general, but it’s a complex topic.
Best advice is to implement a backup solution at the very beginning (even before you start working with openHAB itself). openHABian has a menu option to install/configure Amanda (a backup software solution).
You can use your Fritz!Box as the NAS. If you’re familiar with UNIX, see the link above how to optimize a Pi.