You may want to migrate your UniFi controller to cloud key for about $84 at Baltic network
I run my controller on a similar vintage MacBook without a screen (connected to external display) and will migrate to cloud key soonish.
Ok everything is loaded. I didn't get the USB load to work but I found an Ubuntu 14 mac iso that worked. I burned that to a DVD (yes my drive worked). Then I upgraded everything to 16.0.4.1 server.
As for using a cloud key, that won't work for me as I am using the same device to run an SNMP server. That service once I load all my devices on it will bring the CK to its knees. I will also look to running a Teamspeak server on this box as well.
For those that are thinking about running the CK its about the same hardware as a Pi3.
*Update*
Small lesson learned. If you want to run this unit headless you will need a DVI dummy adapter. I Ordered mine from Amazon and should be here in a couple of days. I'm aware that I could have made one for a couple of dollars but for $3.00 more I can have one that was already put together.
If anyone has any other tips I am open to them.