Using some parts I had lying around, namely an external USB SATA HDD caddy, the HDD from my existing NAS and a few USB cables, I was able to assemble the MK802 NAS - Phase One:
This all runs using the single DC adapter for the external HDD caddy. I am powering the MK802 using the first USB port on the HDD. The second port has a second WiFi adapter plugged in, to allow me to bridge the WiFi interface that will run in AP mode on the MK802, with the existing WiFi router. This will allow me to set up the device with access to the local network and internet (required for updating / downloading packages). All of this is hooked up to the second USB port on the MK802, via the USB input on the HDD caddy, thus allowing the HDD caddy to act as a four port hub too.
The HDD caddy was purchased a few years back on Amazon for around £15 including delivery. It has served me well and makes for a good hot-pluggable HDD caddy.
I have Debian running on the MK802 with a sunxi 3.0.36 kernel. I will provide details on how to set up the MK802 with Debian using an SD card.
Related: MK802 Project - Heat sinks