We are the dedicated kitesurf surf crew in SA. I can't even ride a twini anymore, its too weird, like riding with an anchor tied to an esky lid. Don't get me wrong, I know its not like that but after riding with a purely designed directional strapped kitesurfboard in waves, don't look back.
I also started with a surfboard to get the gybing down, its like learning to surf with a big mal, perfect, so get an old surfboard and start riding, but once the surf is 3 ft, you are going to want something smaller.
Straps or no straps, I ride both, but I prefer straps, nothing beats the hardcore agression that straps provide, nothing.
Cut loose and free your spirit, get a pointy board and get into the waves, you know it.
Yeah baby YEAHHHH
In answer to your question I am 70 kg too and ride a 5'9" Rocket Fish, I have a 5' Freestyle fish and waiting to pick up my 5'10" 2009 Kontact board which replaces the rocket fish from 2008. I would suggest that a kitesurf board around 5'9 - 6' max depending on wind strength. As for surfboard anything 6'6" would be a good start, one with a bit of volume and floatation, say 2 1/2 " deep and 19 - 21 inches wide, but that's just a guide, just don't go too small with the surfboard.
Dazza