I'd go for Logic over Garage Band for sure. +1 for Reaper. Logic isn't being developed. 9 has been out for eternity. Reaper is deep and has a steep learning curve (definitely go for the Groove3 training video course) but rapidly developed and is awesome (disclaimer I use Digital Performer 8 for my primary DAW which I also highly recommend). Plus reaper is the cheapest Tier 1 DAW. I'd also recommend Presonus Studio One and even Reason over Garage Band.
I travel for business extensively. You can use the 1/8th inch out on the macbook. I've done it many times. But I don't recommend it. You can get a small bus powered 2in/2out I/O from many suppliers. What I do these days is carry a Focusrite VRM (virtual reference monitor) and Shure SHR440s, very high quality for only $150 but VRM is output only which suits my needs, and use my MOTU 828 mk3 in the studio. I have the 828 mounted in the small rack as the Axe-FX permanently wired and travel with that extensively as well. If you carry speakers (tiny monitors or headphones) keep them in the cabin. Changing pressure and temperature in the luggage compartment will kill them in no time.