I am thinking you will need more than 2 gallons. I am not sure about the Broadbill, but I have used about 8 gallons on my Cackler.
I ordered my epoxy from US Composits also. The 635 epoxy will work but there are things to consider. I used the slow hardner because I did not want to fiddle around with blush. the 635 epoxy needs warm weather to cure. I personally have a heated work shop and I crank the heat up to about 80 when I am done in there. The epoxy takes about 10 to 14 hours to get really hard. There are faster hardners out there. I am not in a hurry, so I did not care about the time factor.
There were some things I wish would harden up "right now" so I could get to the next step, or so that the epoxy would not run as it was setting. Than can be frustrating at times.
As a rule, I have found that when I glass some part of the boat, the first coat is a heavy one. The cloth soaks up the epoxy. Then I have to put two additional coats on to make the surface smooth. Those coats are not as heavy. All in all, you will use a lot of epoxy if you are like me.
One trick I use is to mix my epoxy in small batches. I use disposable plastic drinking cups. The kind you would buy for a party or the like. I also buy disposable brushes by the case. I use the cup and brush, then I throw it away. That way I never have to clean the brush and it is always nice and soft. For spreading the epoxy around, I use a squeegie as much as possible. for some things it is better than a brush, I think. Get lots of latex gloves. I change mine a lot as I am working through a batch.