Kyran, This is a Sema4Centric view (watching the signs that the seabreezes are coming....

):
RE: Board - Start big but it gets quite choppy on the Gulf so for a smoother ride be prepared to switch down to a smaller board - maybe 40 to 20 L + body wt, as your skill level increases.
Consider sailing on a lake (Goolwa is excellent, West Lakes might play a role but gusty there) initially to get the benefit of flatter water whilst bringing the skill level back up.
For the big board (150L+) consider one with a retractable dagger board if you're planning to teach beginners and children - I've read it avoids them heading straight down wind, car trips to pick them up the other side of the lake, etc....
RE: Sails - a smaller sail on a bigger board is a good combination for learning.
Sema4 sailors generally rigging 4.5 - 6.0 for sea breezes.
RE: Supplier - I use and recommend Ocean Surf Co and SurfSailAustralia
RE: Brand - if you're stoked on a sick board+rig from your favourite brand I say go for it for the psychological advantage it gives you!
Happy safe windsurfing!