How do I add previously purchased music to a playlist on a pc?

Create the playlist, then drag and drop music to the playlist or select the music, right-click > Add to Playlist.

1. Click on the Music icon
2. Click on Playlists - this will open the sidebar that lists all the available playlists in your library
3. Select the items (albums and/or individual songs) that you want to add to the new playlist
4. Drag and drop to the playlist

As an alternative to step 4, you can right-click on the highlighted items, select Add to Playlist and then pick the new playlist from the list displayed:

Note that this works with regular playlists, not with Smart Playlists (which are populated based on the rules defined for them) or for playlist folders (which are just containers for playlists or other playlist folders). In the second method you can see the playlist folders (greyed out), and any smart playlists (identified in the sidebar by the "gear wheel" icon) are not listed.

Yes, on the assumption that your library is associated with the Apple ID that your daughter used to buy the music on her iPhone. In iTunes, select the Store, log in with your Apple ID, then in the Music section on the right-hand side of the screen, click on Purchased. Select Not in My Library at the top of the screen, then click on the Download All button (bottom right).