As I want to buy a new smart tv/ pc monitor soon, what should I purchase - or what tech criteria does it need to have - that will allow me to update the firmware (?) with Ubuntu tv software when it becomes available?
The current plan for Ubuntu TV is to target a 13.04 release. Ubuntu TV is just a different form factor of Ubuntu Unity, so any Ubuntu installation of 13.04 should be able to become a Ubuntu TV interface (and vice versa). To my knowledge (I don't work for Canonical), there aren't any TVs that will run Ubuntu TV that have been announced, and I think installing on current TVs is a bit of a pipe dream (it would require a fair bit of hacking skills to do so, and probably wouldn't run very well).
You could build a box (or buy a small form factor box) that you could plug into a TV via HDMI or similar. Although there is no reference platform, I'd recommend something with an Atom processor. Although it may work on ARM, it isn't planned to be optimized for it so I'm unsure how well it would run.