Bobcat13: Hammocks are unknown territory for me. But my impression is that even a 0 deg bag will do little for you on the bottom of a hammock because of compression and exposure to the air. So my recommendation now would be to get an inexpensive 20 deg bag, then open it up and use it as a quilt. As I understand, hammocks tend to push everything together on top of you so the effect would be to increase loft and warmth (I think), so I see no reason for going to a 0 deg bag with respect to the top of you.

As for insulating the bottom, ask the hammock hangers about that. Probably the best thing is to bring along some extra pads (especially the wider pads) at first and then discard them if you don't need them. I wouldn't spend much money on anything until you really understand the in's and out's of hammocks, since I can easily see how you might want to change everything after some field experience.

A 20 deg bag plus foam pad and a basic headnet is a good idea even with the hammock, since you might be forced to use the shelters in case of problems with the hammock.