As another user who transitioned from a line of notebooks (iBook to 15" MBP, to 13" MBP to 27" iMac) I would recommend getting the magic trackpad. While I primarily use a mouse now days, I still like to use the trackpad for some apps. With more an more iOS gestures coming to OSX over time a user could be missing out on some nifty features with out the trackpad. Other accessories? Lots of external drives, if you don't have them already, I'm working with 7 right now, (2x2TB, 2x1TB, 2x500GB one of which is a 2.5" drive, and 1x 64GB SSD).
As for the second screen, it will hurt your eyes compared to the iMac. I thought I would use my old 22" LCD after getting the iMac, for the same things as you, and the poor performance, in comparison, made me put it back in the box. Not to mention going from the large high resolution screen to a small one with much lower resolution was just painful.
As for buying new machines, since I got a 2011 Core i5 27" iMac that feeling as all but faded. I used to upgrade every two years, but the feeling that I need to do so has all but ended since I moved to this beast. I figure I can get another 2-3 years out this thing at the very least. I'm sure the newer machine are faster, but to be honest I rarely feel held back by what I have now, compared to the notebook I used to own.