I am considering seeing if i can pick up a cheap power meter but i am not sure about a couple of things. I currently run 10spd Tiagra (latest 4700 series) so would i be able to run an Ultergra crank on there (left hand only) and does it matter if its an 11spd crank/power meter like a Stages on Ultegra 5800 crank?

You might be better off with a whole new chainset, they aren’t that much dearer than a single crank with power meter.  Have look for the 4iiii power meter with Praxis cranks – about £500

You’ll also need to check clearance between the crank and the stay.