An interface is something that connects your computer to external devices. In your budget most audio interfaces include ADC, DAC, clock, usually mic/instrument preamps, headphone amps, sometimes even mic (Apogee one). In commercial recording facility budgets many interfaces don't include any of those features, so they use an interface, a DAC, an ADC, a clock, multiple mic preamps, multiple instrument preamps, multiple headphone amps, multiple microphones etc...
I don't know of any dedicated DAC in your budget, and if it's USB/FW then it's an interface (or a DAC with an interface card, which is the same thing as an interface with a DAC included).
Note that you already have a built-in audio interface on your Mac. I just had a quick look at the FiiO E9i and it doesn't seem to bring anything more than what you already have. The Focusrite on the other hand gives you mic preamps - but apparently you don't need them:
My requirements: To mix music in Logic pro, using only my Shure headphones
Plug your Shure headphones into your Mac's headphone output. Done. You'll probably get better results than by using any £100 device.