I use K86 for those sails (85kg). You could use all three Kombat sizes:
89 - probably need more experience than you currently have, and not ideal with 5.9
9x - probably the best choice
10x - if you want more security in the beginning, but less fun with smaller sails and maybe too close to your Aero
I think the best choice for you is a Kombat97(96), ideal for your sail range and chopy/wavey conditions. A word of caution: it's a sinker for 95kg, and you mention cold weather and variable wind. It can be slogged with some experience, but don't get caught far away from shore, unable to waterstart. I still think you should go for it, a little harder in the beginning, but more rewarding in the long run.