I've never owned a MBP prior to buying my 17" a few weeks ago, always owned iMacs. I also bought a refurb 27" ACD. The combination is wonderful.
The ACD is basically an iMac without the logic board and drives. What this means is the ACD includes the speakers, the iSight camera, the internal mic, 3 USB ports, and of course the screen. An added feature is a cord that provides power to the MBP, USB plug, and the video/audio plug.
My MBP sits vertical in BookArc in clamshell mode. Essentially the MBP is providing the logic board and video.
The combo is like having an iMac, except I can have portability by simply removing the three connectors from the ACD.
It will definitely take up more space than an iMac because the MBP has to sit somewhere, even in clamshell mode.
The keyboard is an Apple wireless & Magic Trackpad. Another wonderful combo.
Buying only what you need, or can get by with is admirable, but when you're going to spend the money for the best computer anyway, buying what you would like to have just makes the purchase a LOT more satisfying. However, if you just view your equipment as another tool, then just getting by is probably fine.