I live in Scarborough, and work right next to the hospital, journey time is around 25 minutes if you go the right way

Cottesloe is probably a bit quicker, maybe 20 minutes.
Cripes, 65 kilos? Your 75 will be fine. I normally ride my 80 litre wave board for waves, I weigh 85 kilos... I can't suggest a sail size, not having ever windsurfed when I was 65 kilos

Scarborough is a beach break, so if you catch a wave you get one or two bottom turns before the wave closes out. There are other spots around if you're willing to look though. For windsurfing it's got a vicious shorebreak, be prepared to get worked and look like a beginner

Dutch Inn is much more sedate and easier to get going.
You might get a sail in after work if you're quick, but you won't be able to stay out there long. The sun is setting around 6pm at the moment, expect it to set earlier by the time you get here. As Rider5 said, in winter you take the wind when it comes, if it comes in at 9am it might be gone by 4pm.
Good luck, see you on the water