First, sorry for my bad english.
I'm too a first generation windsurfer, last year i bought a Rio M, is a very good board for learning, nothing more. If you plan that your whole family learn windsurf, it will be a good board choice.
You can not compare a Rio with a Go (any Go) the Rio is a heavy board and also in planning mode, it seems to stick in the water.
I'm also 85 kgs, I think for low wind the Go 155 will be a good choice. I have one, is very stable, friendly for learning, early planning and fast, it will grow with you.