Since I teach beginners and the majority of them are women and girls, I think you are close to correct here.
I see you wish to purcahse used gear and have a budget.
That can prove to be a huge problem.
Smaller women and girls really need the lightest windsurfing rigs you can buy...regardless of cost.
Buy her something in that size range that's heavy...designed for high winds or wavesailing, and she will hate it because it has no power in 5-10 knots of wind, and actually... it's just too heavy.
My suggestion would be something like a 4.2 meter Sailworks Retro Ripper on a 370 cm Sailworks Backbone trainer mast. Get a light weight trainer boom to go with it and she will love it and progress very quickly.
These sails actually have remarkable power (for their size) and work well in your stated windspeeds.