The 30cm fin has a quite powerful profile and at your weight it should handle 7.0. I used the 28cm Drake crossover fin with 7.0 myself (on a flare). IN any case you can try it first before buying something new.
When it comes to small sails, it will depend a little on the water state. If it is not too choppy and if you stay away from sailing too overpowered the K103 can probably handle 5.0. But its for sure a pretty big board for you in under 5.0 conditions.
But again, the best thing is trying. And if it just feel to hectic on the windy days, you can complement with a wave board in the 75 liter range. A classic wave board from the last 4-5 years can often be found at a very good price and at your weight most such boards will work perfect as high wind freeriders and be much easier that one might think to sail as long as you're powered up.