Biab is AG, you just add all your water at the beginning ( unless you are Maxi-Biabing which is a variant to get more beer out of your kettle than a full volume would give you)
You will need a boiler of some discription, size depends upon how much you want to brew but the rule of thumb is the bigger the better. I full volume biab and my pot is 52L.
If your looking at the burco type boiler which is probably around 19L you are on the right track as loads of biabers use those.
I'm assuming you have been reading the Brew in a Bag section on here. Another good resource is
Here