Hi, Chris. iTunes should be able to do what you want.
First, get your songs organized in iTunes so that you can select all of the ones that you want to convert. You can create a Playlist, or just have them all in your library.
Next, go into "Edit - Preferences - General" and change the "Import Settings" to MP3 and the bit rate that you want.
Next, go into "Edit - Preferences - Advanced" and change the iTunes media folder location to something else. This will be the location where it will store your new MP3 files.
Then, select all of the files to convert, right click, and choose "Create MP3 Version." It will convert all of them, except for any protected AAC files that can't be converted.