the more ram you got, the better. I have it on one phone with only 512 MB of Ram, and CPU 2x1.3GHz. CPU is ok. XFCE with Iceweasel struggles with 2-3 tabs. So it is only useful for GUI-less tasks. The second phone I use it on - has Snapdragon 800 - 4x2.3GHz and 2GB RAM. Debian with XFCE flies on it.

Well I have two candidate devices, a Z3 and a Samsung galaxy tab s2. Is there a way to hook up a second screen somehow?

Anything that works for your phone - hdmi, mhl, widi?

beware of 2 mhl specs. 5pin/11pin. Look very similar. I did not read it very carefully, so now I own both

haven't tried yet, i think samsung sells an hdmi thing that needs power via usb and connects to the galaxy tab micro usb port.

Question is whether the tablet could only mirror it's own resolution on linux, or run at a higher monitor resolution, with usb connected mouse and keyboard. That's make it a really useful device all of a sudden.