For the filler on the inner bond rail gap, you can use either foam or epoxy based filler. The filler is going to be stronger, but require more prep and add more weight. 4lb foam would save you weight, reduce the prep time, but also provide less strength. Considering you have a broken rail, the filler might be the better choice. If it were my ski, which isn't broken, I'd probably use foam, but your ski is broken.
The filler you make yourself. Buy Glass Microspeheres and Cab-o-sil. Start with a 50/50 mix of the two and mix to a peanutbutter consistancy. You may change the microsphere/cabosil mix some to either get better consistancy or better sandability. The microspheres make the mix more sandable, the cabosil makes the mix more of the consistancy you want. Note: When I say 50/50, I am talking strictly about these 2 parts, there are 3 parts to the mix, obviously the epoxy, that is excluded from my 50/50 mix directions.
I personnaly work with about 2-4 oz of epoxy at a time. I add the fillers until I get the mix I want. Note that 2oz of epoxy will yield 6-8 oz of finished filler. DO NOT add too much filler and dry out the mix. You want peanutbutter or a little wetter.
As far as the glass, I can't make a specific recommendation as I have not done that job. There are 3 kinds of glass. Chop, woven (mat) and biax (combo). Biax is the only one considered to be structural glass. I almost exclusively use biax.
More thin layers are stronger than fewer thicker layers. Thicker cloth is also brutal on your scissors. I tend to work more with a 12 oz biax over the 17 oz boax (ie. 3 layers of 12 over 2 layers of 17). The thinner stuff is also easier to work. However, it is also more time consuming. When I work with heavier cloth, it is when I am trying to save time, such as when I did my X2 hood, and I needed a lot of strength around the steering. I used heavier glass to get the job done quicker (I went like 1/4" thick around the steering), but that doesn't apply to you. I do not know how much glass you should do for this job... Weight is more important than strength to me, I'm not going that big, so I have no reinforcing anywhere.
Disclosure as normal... I am not an expert. You are responsible for making sure you do your homework.