The MBA is a great platform for this. I used my 2011 13" i5/4gb/256gb MBA before and last summer upgraded to a 2013 11" i7/8gb/512gb MBA. Recording directly to the SSD with the 4 channel Akai EIE Pro interface, my 2013 machine shows 9ms roundtrip latency; the 2011 MBA showed 13ms latency with the same setup. Guessing that the blazing fast 512gb SSD on the 2013 machine is the main factor there.
I have been using Logic Pro 9 and like it a lot. But I used GarageBand on the old MBA too and it is surprisingly powerful and easy to use. I just prefer the Logic user interface better.
How are you using the mixer? How do you "output to a mic and guitar"?

Do you mean "input from a mic and guitar" maybe? It is better not to use a mixer IMO, because you won't be able to change the balance between the mic and guitar after recording. I prefer using an audio interface with separate inputs - that gives you more control after recording. You would then use software for your "mixing".
Take a look at the Korg nanoKontrol2. Inexpensive control surface that gives you 8 faders with solo/mute/record buttons and pan knobs plus transport controls. This will behave like a "mixer" but it's all done in software. Just download the Mac driver from Korg and it works right out of the box with Garageband and Logic. Best $60 I ever spent.
http://www.sweetwater.com/store/detail/nanoKON2bk