Once you remove the downpipe, the manifold will be easy to remove with one caveat: the nuts may be stuck onto the studs. For a few days in advance, soak these nuts with PBlaster and follow instructions in the FAQ. Use a six-point socket to remove them and reinstall using new nuts with nickel-based high-temperature antiseize. The studs can remain if they are not broken.
The muffler is virtually indestructible and is fine.
The oxygen sensor wrench is easily found at Autozone: it is a socket with a slot up the side for the pigtail wire. You can rent one or buy it for $8 or so. See the FAQ for tips on wiring.
This project will not ruin Christmas unless you start it on Dec 24th.