The Sample Rate and Bit Depth are under Edit → Preferences → Audio Settings.
Leave the bit depth at 32-bit float. This is what Audacity is “happy” using internally.
You can set the bit depth when you export as WAV. 24-bits is fine for your temporary-working files. I always recommend WAV back-ups whether you save an Audacity project file or not. And of course, multiple backups in different locations depending on the importance of the recording.
You’ll need to submit an MP3 to ACX. MP3 doesn’t have a bit depth. It has a sample rate and a bit rate. The ACX Requirement is a bitrate of 192kbps or higher.
In case you don’t already have it, there is a macro here that will nail the RMS & peak level requirements. You should also have ACX Check.