Darren,
Just some support on what lee1972 said, if you spend a little extra and get a new (superseeded)/near new big shortboard ie shark 160 you can basically upgrade as you get better with little or no changeover in $. Any of these big short boards keep their value extremely well, or if you can see into your windsurfing future a bit they can be something you will never part with. Sails, booms, bases, fins and slalom boards pretty much loose a third- half their value when they walk out of the shop- big freeride boards and most masts you won't loose too much on should you need to sell or upgrade later