It would be better if you had a little more experience, and I really think the Rio M might be better for your conditions on fresh water, but the Go 151 is a very close 2nd choice in my opinion.
Yes, the center fin will enable you to go upwind in very light winds, but as soon as you are in enough wind to plane, you will have to take the center fin out and figure out how to stay upwind on the rear fin only.
You will have to learn to "rail" the GO 151 without the centerfin if you want to progress, but you could learn the same thing on the Rio M.
On the Rio M you have a retractable centerboard so you can ease into the transition from longboard to short board technique a bit more smoothly.
Sail size for 85 KG (187 lbs.) on the Go 151 for a virtual beginner?
I'd suggest getting a 6.0-6.5 m2 as your small sail, and a 7.5 a little later in your learning curve.
Yes, with good technique you will be able to uphaul te GO 151 at your weight.
The volume is marginal for a person with bare beginner skills.
Your kids will be able to use the GO 151 quite nicely, but you will need to purchase a seperate kids rig for them. I would not go below 2.5 m2 as with the fixed centerboard on the GO 151, they may not get any upwind drive with rigs < 2.5 m2.
If you choose carefully you should be able to find 6.0-7.5 sails that will rig on the same 460 cm IMCS
24-26 mast and a good adjustable freeride or slalom size boom.
Hope this helps,