How strong is the wind you will be sailing in?
@ 65 Kg. (143 lbs.), on the S Type 115 with a 5.0 m2 rig you would need 16-20 knots to even get planing
at all. That's Bft F4-5 or around 30 k/h to 37 k/h.
If you have at least 16 knots/F4.5/33.5 k/h then the 5.0 m2 rig will work, but if you have less wind than this,
the S-Type 115 is not going to be much fun to slog around on.
Hope this helps,