The issue with ventilation will not get better with soffit vents only, in fact it gets worse.You let in miosture that cannot get out. If you cannot get any top venting in, just go with the foam board insulation in the porch. I know its a little more expensive but it will hold up longer.Spray foam around any gaps and make sure you hold it off the underside of the decking/roof material (plywood)at least 3/4 inch.
Try to stay away from plastic vapor barriers , they are nothing but trouble.Use the kraft faced paper as I said before.Staple it to the face of the studs,not the sides.
Update any electrical now.
What are you doing for heat? Hope it's enough with all those windows.