Ubiquiti Unifi runs fine on hardware as modest as a Raspberry Pi 2, on Debian or Ubuntu.
run the commands
echo "deb http://www.ubnt.com/downloads/unifi/debian unifi5 ubiquiti" > /etc/apt/sources.list.d/ubnt.list apt-key adv --keyserver keyserver.ubuntu.com --recv C0A52C50 apt update apt install unifi
after about 10 seconds browse https://localhost:8443 – note the https is required. If you have a very slow CPU (slower than Raspberry Pi 2) you may need to wait a little longer.
Java 8 is recommended for Unifi >= 5.3, and at least Java 7 is required.