With magnets that close together you are wasting a lot of winding space.
In this case it doesn't matter as you have a more powerful alternator than you need.
If you want to save some money I don't think you loose anything by using 20 magnets per disc and making better use of the space.
I wouldn't take the cut in below 80 rpm, it will make things much more difficult.
I think you can do it with 24 magnets with 80 turns per coil, the wire will be incredibly thick and you will have to use several strands of #14 or similar in hand.
You will need to keep the coils over an inch thick for those magnets to make good use of the winding area, up to 1 1/8" thick would be ok.
It will be badly stalled by 15 mph and you will have to add line heaters to keep the speed up, but this does mean that the dissipated heat is not in the stator.
On windy days you may be able to cut the line resistor out and let the thing trundle quietly and peacefully in stall and still do what you want.
It's a big venture, I am sure you can do it, I can offer help but it is way beyond my experience so you must check things for yourself and make your own final decisions.
Flux