@charlag Try to get as much RAM as you can. If you want to run a Matrix homeserver then that will consume at least 1 Gb of RAM. Most other apps take a lot less though.

Although not the latest and greatest a Cubietruck of Olinuxino Lime2 should be ok. If you don't care about the proprietary boot blob then you could go for one of the aarch64 octacore boards which would make a really powerful server.

https://linux-sunxi.org/Olimex_A20-OLinuXino-Lime2

http://www.hardkernel.com/main/main.php