Wow. You have in your hands some of the most advanced and powerful music production software on the planet. Surely you would want an audio interface to make it sing rather than limp along. How are you going to control momitor level, headphone level, etc. If you are going to connect a mic, it is essential (unles you are going to opt for a USB mic. There are very few pros who would choose that option). Any audio interface will sound better than the sound card inside the mac, but if you go for something like Native Instruments Komlete 6, it's affordable, user friendly and sounds amazing. There are other good ones, like Focusrite and MOTU, but M audio and the lower range of Presonus are not in the same league and the more advanced users would avoid them.
Hope this helps.