I've had the S-type 115 (2005 model), and it's one of the fastest and greatest sailing boards I've ever had. It's not a big sail carrier (7.5 max for my 95 kgs) or early planing board though.
My iSonic 122 will carry more sail but surprisingly the S-type clocked a higher max speed.
At your weight (54 kgs) the 104 will be great. Why do you need + 60 litres? You won't carry more than 7.5 metre on the 115 and the 104 will be best with 7.0 as absolutely max.
I did + 30 knots easily on the 115 with a 5.7 sail, and surprisingly it worked with a 4.7.
The 104 will be great with your sails (except 7.5).
I've had an ST 126 as well and it didn't work with anything bigger than 8.0.. Actually it didn't really work for me at all...