1)-The planned upgrade brings you a nice improvement in terms of screen real state. Use of palettes and menus can be better, for example.Thing with iMac is, are tools for color correction? Same thing with the monitor you mention.
2-How well your current workflow/peripherals integrate with the iMac. Can be enhaced and your production improves?
Hope my post helps you think and consider more wider issues.Not saying your original considerations/thinking are not worthwhile.
The iMac is a better computer with or without a screen.
NEC makes multiple models with the PA series (perhaps there is a newer series) that are meant to be used for graphics and thus intended for proper calibration from the get go. You didn't say which model so no way to know if its worth the price.
Obviously if its the graphics version and you have the bucks, get both the iMac and the NEC.
I use a Mac Mini quad 2.0 with a 24" NEC PA24IW and with the 16 gigs of RAM, it works well for photo work. Your mileage may vary.