The i7 imac is more than enough for prepress or design. The Pro is way overkill and to much of a machine anyways, you probably don't need all the expansion and it costs a lot more.
As far as the glossy screen, just buy another monitor and run dual monitors, its probably cheaper than paying someone to change out the glass (or maybe not) i don't know what it costs.
As far as i can tell the i7 iMac runs circles around the current high end Pro machine in a lot of benchmarks (not all) so i am sure you will be happy with it and the price.
FYI - APPLE IS HAVING PROBLEMS WITH THE 27" SCREEN SO BE CAREFUL, GO TO
Mac Rumors: Apple Mac Rumors and News You Care About TO READ ABOUT THE 27" SCREEN ISSUES.