I actually agree 100% with Peter's opinion, give or take a knot.
NO kite can perform under 7-8 knots. A lot of lies are told about the light wind abilities of various kites, especially by Flysurfer ram air lovers, but NO kite will perform under 7-8 knots.
The best light wind kites are old 20m inflatables like the 20m Waroo or Rhino 2 or 6.
What is the lowest wind condition I can go kitesurfing and what is the best kite brand and size for it?
It depends a lot on your weight, how low you can go.
If you are "average" around 80kg, the lowest you can go will be around 5m/s (10knots) with the right kite and board.
If you are REALLY experienced, you will be able to push the lower limit down to about 8-9 knots, but this is the absolute lowest wind possible to kitesurf in (holding ground/going upwind) with average weight IMO.
We are talking about 14-17m2 typical SLE kites and raceboards here
And talking about EXACTLY how "low you can go" is just pure bullshit - as you can not measure the windspeed at the kite, which is the only true value for this.
Sometimes you have a huge windgradient, sometimes a small one, and air temperature and height also influences.
But around 10knots is the limit for most kitesurfers, and just a small tad lower for the "extreme" ones
When you talk about windspeed - where is it measured then ?
At headheight, maybe around 2 meter above the water ?
Or at 10 meter height, which is our (Denmark) meterological standard height for wind measurements ?
There is a difference of typical 2 knots, so VERY important.
My point is - always take those claiming "this and that" as their minimum wind speed with a grain of salt
But the answer to your question would IMO be around or just below 10 knots measured at 2 meters height (in normal "summer" temperatures), and it is not dependant on brand, as many brands has excellent kites for this - and more and more has raceboards now too.
I can understand you dont kitesurf yet - but interested in the low level ?
Typically, you can start just a tad earlier than a really skilled 12m2 formula windsurfer, but almost the same planing level - maybe this makes more sense to you if used to windsurfing ?
Hope this helps