:D Congrats on getting the go-ahead for a pond! They can be lots and lots of fun. First I'll address the temp issues... It is too cold right now to just stick her outside. A temp shock like that has a good chance of killing her. Goldfish can take temps down to freezing, but they need to be slowly acclimated over weeks. Because we've had this cold snap (I'm north of you in southern Virginia) it's not going to be a good idea to put her outside right now. I was planning to do a small tub pond this year and have had to put those plans on hold, too. You should wait until nighttime temps don't go below 60F before putting her outside. In small containers the water temp will drop much faster than in large ones.
I'll answer your questions here to let others see them and add their input.
Is there a certain brand/type of plastic I should avoid?
You want plastic that is safe to hold food for human consumption. Pretty sure Sterilite bins are safe. For the actual pond, you should look at prefabricated liners. Shoot for something between 150 and 300 gallons. The larger you go, the larger your goldfish will get!
Will the temperature change kill her? Nights are supposed to be in the high forties and low fifties here in GA for the next week or more.
Right now, she could die from that temp change. Tanks inside typically don't go below 60F inside and a rapid drop in temp like she would experience in a small tub outside would be detrimental to her health. This cold snap is going to put a dent in your plans unfortunately, but that's just part of the game with ponds and tubs outside. We aren't in charge... mother nature is.
What kind of filter should I put in there?
In her little tub, something as simple as a sponge filter would work. Or even just a bubbler for water movement if you can score some floating pond plants like water lettuce, duckweed, or water hyacinth. For the pond, most people have waterfall filters. The water is pumped from the bottom of the pond up through a hose into a container above the pond and flows back over rocks.
How often should I clean the water, and how much water should I change at a time?
I'd say change the water once a week doing about 50% water changes. So as not to send her into shock, pour all of the water from her little tank into the tub and then fill the rest up with dechlorinated water. With your pond, you won't need to do regular water changes. Again mother nature takes over. When it rains, the pond will overflow (some have overflow spouts built into them so it doesn't go everywhere) and drive out the old water. The rain coming into the pond is the fresh water.
Should I put in plants and decorations?
Put all of the gravel and stuff from her old tank into the tub. These have beneficial bacteria that will aid in cycling the tub super quick. I'm a live plant fiend. Always live plants!!! Floating plants will be best both in the tub and pond. Marginal plants like iris and cattails are great in a pond. Iris add lovely color and cattails grow rather fast and help suck up nutrients. You'll have to see what's available in your area. The neat thing about ponds is that you become both gardener and fishkeeper!
The last bit of info I can give you is to look for local pond clubs. These people have ponds in your area and will know the little ins and outs such as just how deep your pond needs to be to escape freezing over in the winter. And a lot of times they will give you extra plants. Pond plants need to be trimmed just like aquarium plants and some people don't like just throwing those plants away. Most the the plants in my pond were gifts from other pondkeepers in the area!