I'm a soundguy and I work as a semi-professional (I have also another job...) in my area, nothing fancy but pretty serious. I work with Logic platform only.
My uninterested advice, if the only thing you want is record one track at a time, and compose a mix with a simple DAW (digital audio workstation) like Garageband, well, you only need your iMac and nothing else. The stock built-in audio interface is pretty good on desktop macs, it should work with a 24 bit depth and go above 90 Khz sample rate, double the rate required to write a musical CD with .aiff or .wave audio formats...
The quality of the stock A/D converters is acceptable for non-pro user, and if I were a "less than professional" user, I would stick with the built-in one.
Myself, I prefer superior audio quality and I have external audio interfaces, because I need simultaneous multi-tracking and so on, but if you enter this game, you may discover an endless world, too vaste compared to your common needs.
I must admit that, sometimes, I produce demos for little local bands and I just use a Macbook with good RAM and a fast HD, stock audio interface, more than basic results if you know how to mix and edit...