Righto, well here's some comparative information for you.
My brother rides the zephyr and he's about 77 to 79kg, slightly lighter than you. He has a 145 by 44 crazy fly all round board. Now he can just stay upwind in 10 knots, without current on flat water...to be quite honest he wouldn't bother and he has some light wind voodoo going on that guy, so if you want down to 10...this gets harder. You are really getting into the need for a very efficient board ...eg sector, nugget, large SB...
At 12 he's doing okay, can throw some stuff down but not a great deal, but staying upwind in a reasonable fashion. 13 up he's in his element, then by 15 changing down board size u p to 20 knots then it's kite change time. But he does like to be powered so take that into consideration. He could be on his 136 by 41 raptor pro at 13/14 if he so wished.
From this, I recon you are looking for at least something 46 wide, but no wider than 48 if you want to 'get busy'. Another guy I ride with has a 48 and he's a really powered rider who likes all that darkslide etc jazz, and he wouldn't go any wider.
So then it's down to length and rocker, and overall outline. I run a crazy fly cruiser pro, 145 by 44 at 74-75kg, which is reasonably square in outline and not a great deal of rocker. But the length does give it enough flex. The all round is a little rounder in outline and has more rocker and is definetly more comfortable but not as good for load and pop.
Anyhow not vouching for any boards or kites in particular just giving you some kind of yardsticks to add into your decision making mix. Hope this helps along with other useful information here, except for freeze of course.