Do you think you need a DAC? Are you unhappy with the quality of your sound? Do you listen to lossless codec's and can hear the difference between say MP3 and FLAC or ALAC?
If you really want high quality sound, a DAC and headphone amp are a good investment, but if you're listening to low quality codec's then you might be wasting money on a DAC/amp.
The impedance shouldn’t be too much for the iMacs headphones port. So if you just need headphones for Apple Music / Spotify and YouTube you should be fine.
If you listen to more dynamic movies and uncompressed audio a headphone amp with DAC would make sense.
There is also a 32 ohm version of the 770 pro but it sounds a bit harsher compared to the 80 and 250 ohm version.