I would highly recommend the apogee duet as an audio interface:-
http://www.apogeedigital.com/products/duet.php
In my experience, music programs such as Logic, ProTools etc do not rely very much at all on CPU speed - it is RAM which limits me rather than anything else -so make sure you get plenty of it.
That said, any MBP should run the software you need excellently - even a macbook would probably be adequate if apple hadn't removed firewire.