If you already surf then it should be a breeze for you, the once you are up riding bit will be easy, start off in knee deep water stepping onto the board before practising deep water starts. As you already have a twinny i would advise at least a couple off sessions on that so you get used to the physics of starting, it will help you use the fin when you move onto the surf. I wouldnt bother waiting until you can ride upwind, just use the twinny to get the basics of getting up and riding as the way you ride a twinny and a surfboard are completely different in respect of kite power and useage so you might just pick up bad habits. I hate to say it but think about some kind of safe leash while learning as everytime you mess up the board will tend to return to the beach if your not quick, if this is through the line up the locals might become ex mates. Also when you fall, jump clear and try to break the habit of falling on to the board as your hook will cause a bit of a dent
Boards wise, anything you paddle would be good, a bic 6'3 pop out is pretty indestructable but a little bit bouncy, good to learn on and you would have no bother shifting it in the summer, local custom stuff around 6 foot, i ride a 5'9 and weigh a bit more but i need to be quite powered to ride it, or lose some weight.