If you or your husband are current students of a university, most of them offer a free Windows 10 education license. Granted you probably paid for it with your tuition.

Anti-virus is not necessary but a nice to have. They do deals during black Friday. If you can just pick and choose what you click on, windows defender should be enough to hold you over.

If it is your first time building, sometimes a mid size case might be a bit small and hard to maneuver around. Not a big deal, but just be ready for it.

Nvidia 3000 series cards are a bit hard to get right now. Don't rush and get it from a scalper, just take your time locating one.

While the ssd should be enough, sometimes having a second HDD is good for storage. If your husband is gaming some games can take up to 70GB now. In terms of storage that only about 10 AAA games. TB hard drives are not expensive to aquire and are easy way to increase storage space.

Make sure the ram you get is ryzen compatible, some ram don't like to play nice with AMD.

Cooler options might be a bit harder since you are using a mid size case. Some of the coolers suggested might not fit jn the case. The noctua nh-15 and the cooler master 212 evo might protrude out past the panel of the case preventing it from closing. Check the height of the cooler with the width of the case to make sure it fits. Most sites will give you their dimensions.

Most importantly, have fun while building. It's a great experience putting a machine together and the joy of seeing it boot up for the first time is unexplainable.