Thank you biothunder for your insightful suggestions! I will get the Molex T splitter cable and attach a 12V fan to it. I hope you get the system up as well, the Ubuntu guide worked for me, except one pecularity: The u-boot ext4 reader mixes up /boot and /var so I had to use:
Marvell>> setenv image_name var/Image
Marvell>> setenv fdt_name var/armada-3720-community.dtb
