I would say use new gaskets, it is basicaly a round packing gland that fits in recess...yeah you can reuse them, but if you have time to order them why take the risk over a few dollars savings when replacing fresh clean surfaces to start?
Regarding the O2 sensors, here is a Self Help tip:
Take your time putting the new exhaust gaskets in. If they get out of round or a banged up edge, they can be a real hastle to install. A pick took works well to get the old ones out.