Just promise you will not take whatever 12m you buy out over 15 knots for the first many many times you kite. So if you plan to kite in higher winds for godsake get a 7m as well. See last few safety related posts re. beginners with big kites.
15 knot maximum? For a 12m? I think that's a little overdoing it with the safety (and I tend to err on the safe side). It depends on the size of the rider and the model of the kite, but I think up to 20 knots is plenty safe for a 12m.
North days up to 27 knots for the 12m Rebel, but I agree that would be too much for an average sized beginner.