Well, by default, the A1200 outputs at 15khz modes. That monitor would need a scandoubler to see those modes.
Now, if you ONLY want workbench, you can hook up 15khz device, (so you can see) and copy the "VGAOnly" file to your monitors drawer. Then shut the system down, connect the NEC monitor, and then power up. The Amiga Workbench will then come up in a mode that can be displayed on that monitor. Of course, there is a large selection of software (mostly games) that won't work properly like this.