Thanks omg; for the 12m and your weight, the 32 gris ok. My actual P12m is 32gr , and even with my 61 kg, it works well in my own lower end (even if I must say I am in love with my former 27gr, more rigid, easier to loop upward but less watertight and maybe robust); 32gr is now the frequent cloth used for the 12m from the user feedback I was able to gather. But do not worry too much, I know a 85 kg guy who owns the P15 in 40gr and makes it fly better than average foilkites.
Now low end P12 vs P18. Not straight answer , but for a hydrofoil application, I would say:
a- below 55/60 kg, I would say it is the same in term of planning and stay upwind (=5/6 knots, it correspond to hear the sound of the wind in both ears). However once foiling up P18 will go faster and accept lower lulls like glassy local areas. On the other side, if wind is irregular like on lakes, P12 will be more efficient and 'workable'. Needless to say that to me the P18 has to be in 27gr.
b- between 60 kg and 70/75 kg: P18 will start a bit sooner (=5/6 knots) vs P12 (6/7 knots); 27 gr cloth recommended for the P18. In may personal case ( regular wind and large foil wing), the optimum planning seems P15m but with less speed than P18 when launched. I am currently trying new waterstart methods with the P18 to try to push the limit further down.
c- above 70/75 kg, a significant gain in low end with the P18 (=6 knots, as it has a ton of power that will be necessary for a heavy rider) vs P12 ( 7 knots minimum); you seem in this category ; however in irregular winds or tricky launch areas, P12 will be easier to manage . A good compromise may be the P15m that I liked a lot and sold it by mistake ...
(*) I put knots values just to illustrate my thoughts ; to be use in relative comparision, not as absolute values too hard to measure accurately.
Generally speaking for foilkite users, P12 is more powerfull than similar size foilkites (and less comfortable in high wind); For example, a good friend, very skilled in light wind, had the same low end as his S4 Lotus 15m which was a reference.