Glad to be of service.

Yeah just think of audio interfaces as external sound cards with all sorts of inputs and outputs. You will also need one if you ever plan to get a pair of studio monitor speakers down the line.

For now though I think you'd be just fine with only the MIDI controller and your computer. That will get you in pretty well.

If you want, also look into getting a midi controller that also has some knobs and faders on it. Logic has a learn feature (command L) that will sync those knobs on your midi keyboard to control the on screen pan, volume, sends, etc knobs in logic. It's almost like having a small mixing board on your midi keyboard!