You'll probably only use your computer for what you're doing now, your own personal work, cutting reels, small side projects, and maybe some independent work, once you start working in the industry. In my experience, it's a very, VERY rare thing to be doing professional work from home. Usually, they want you in house, not in YOUR house. If you want to make a feature film or something like that, make sure you know exactly what your image acquisition is going to be ahead of time so you can design a good workflow. Post here if this is the case, and we'll help you. Or, post on the Cow.
In terms of MoGraph, I'll point to one of my best friends who has worked as a compositor, AE, and Editor in LA for YEARS, and does the best MoGraph of anyone I've ever known. He does all his own work on last years' iMac, which has included some heavy things, and has never had trouble with it.
Regarding the notion that only HD-SDI from an AJA device is sufficient for monitoring and grading, I'll remind everyone involved in this discussion that the quality of your monitoring output is only as good as the quality and calibratability of your monitor. If he's outputting through a Kona3 (or AJA IO, which IS a badass box) to a Vizio 720P monitor from Costco, there's no point, as the integrated display on the iMac is going to be FAR more accurate. In my own work as a colorist in LA, I've often relied on the Apple Cinema display as a secondary check, even though I was using a $50,000 calibrated 2k display through the Kona 3 card. Dunno why, that display always just looked a little rosy, to my eyes, Sonys tend to. The iMac has an excellent integrated display, so when you do color grading (make sure to find a second external monitor to use Color, because I don't think you can even use the app with one), turn out the lights, do it at night, and make sure you've got your built-in monitor as accurately calibrated as you can, check it with bars both in FCP and Color.
Ok, that was more than one thing. But yeah. Get the iMac. If you need a Pro later, you can just get a Pro later. Hell, you could even keep the iMac and use it as a display/render node.