Any wind range is suitable for windsurfing, depending on the gear you're using and what you want to do. ie cruising about on a big floaty board you can go anywhere between 2-15kts, and planing conditions (on smaller boards) generally exist when the wind is 15kts+
You're probably better off discussing lessons (cost and number) with an instructor as they'll be more suited to discussing/assessing your personal requirements. Checkout the link at the bottom of this post for a list of windsurfing schools in WA.
Perth is probably the best place in Australia to buy second-hand windsurfing gear.
Have a look through the classifieds here on seabreeze to get an idea of the range of equipment available. I think the smartest option would be to visit the shops around here, seek their advice and ask about a package deal, even for the 2nd-hand stuff. Maybe even consider a package deal with the lessons??
I only got back into the sport about a year ago and I was somewhere between the beginner and intermediate stage. I found most shops pretty friendly and helpful, and managed to get exactly what I wanted. I ended up with a mix of new and used gear for less than $1500, and if I'd wanted to wait I could have possibly saved some money buying privately, but then I could have also bought un-suitable equipment... I'm still using all of the stuff I bought originally, but as I've progressed I've added more to my collection, so I can mix and match a bit.
This link gives you a list of instructors and shops around Perth.
www.windsurfing.org/wa.htmHope that helps!