12m switchblade should be plenty for your 78k kegs........Im 100+ and have a 12m, does me fine most of the time. And yes, the season is comming to an end and has been poor, with little or no wind, and very frustrating.
I would recommend holding off on purchasing a second kite untill you get some time on the water under your belt, riding comfortably, staying upwind etc. By this time you should have enough understanding of your needs and some experience to be able to make some sort of informed decision.
Patience Tom, it sounds as though you have only had one lesson, and already got your gear, lack of wind is frustrating, but pick your days and make the most of those opportunities. I doubt anyone here would be advising you to get a bigger kite at this stage......maybe a wetty for the upcomming winter months

Good luck.