Make sure to get the LAME plugin so you can not only edit but save out the mp3 files you're working on; without the plugin you'll have to work on the files as mp3 but only save as WAV which would then require some way to convert back to mp3.
With the LAME plugin, all that is handled for you - just make sure to set the options for the mp3 encoder when you use it to keep the bitrates identical.