I don't work for your dad.
Luckily though, I have both a Mac Pro and a MacBook Pro (which am now using hooked up via TB to my 24" LCD). The last job I worked had an iMac I was forced to use, so I can give an honest opinion on all the solutions as ive been exposed to a lot in my time. Let me tell you from the perspective of a professional;
The iMac in a studio environment was the least pleasant experience on any machine I've worked on. The screen was crap to the point my ditzy coworker used as a mirror, the graphics card inside was the equivalent garbage to that of a mobile gpu forced into a desktop where it has no godly place being, I could not install a secondary hard drive for greater storage of files and when working on intensive documents, regardless of being in photoshop or not, it was a slow tragedy. And if you think Indesign doesnt require a good computer, you might want to talk to your dad about CS5, its by far the worst piece of software in the entire Creative Suite there is. I don't mean that by functionality, I mean that in performance.
Don't take this the wrong way but who cares about your issues? Most probably your workflow needs a Mac Pro. Nothing in my father's studio requires a Mac Pro, and even though couple of Mac Pro's might speed up some of the workflow in an insignificant way, the extra cost wouldn't justify it.
I never claimed that iMac can do the job for everyone. I'm just pissed about the ignorants who think "if you are a professional, then you need a Mac Pro".
That hasn't been true for a long time now. Professional market has been switching to iMac/Mini solutions for a while.