One other thing to consider: if you ever want to do any color grading, you simply can’t do it on an iMac or a MacBook Pro. You MUST use an external, broadcast spec monitor, which requires a broadcast video board (AJA or BlackMagic) and that will require a Mac Pro.
As far as I’m concerned, the only use for an iMac or MacBook Pro with regards to editing is AS AN ADDITION TO a fully kitted out Mac Pro workstation. iMacs work well to enable you to start cutting another project while your Mac Pro is busy compressing. A MacBook Pro can work in the field to ingest media directly from a camera and to start editing a project on a plane on your way somewhere. Trying to use either of those as a complete suite will only lead to frustration.