Hmmm Tom, after reading this I am not so sure it is the right choice for you, based on my experience viewing how others "do" around here...
For me, a 12 m2 would only be used in 2-3 m/s wind, and definitely too much for me when 4, so not possible to make tight fun turns anymore (only if you shorten the lines, yes, but then it can not be used in less than 3 m/s)
So a 9 or 10 m2 is a good choice for a "one foilkite" to fill the largest possible windrange IMO, just as you were thinking.
Dont forget, most that uses foilkites like to ride powered and go fast (not all, but definitely far most), and doing carves back and fourth in chop waves is not their thing
These kites got a good upper end too indeed, but not if you want to play around, where too lazy.
Their ultimate low end is far better because of their powerspike.
You might love foilkites, when you get one and learn to ride it (it is NOT like you are used to, at all), or you will hate it and sell it again - time will tell