With the A1200 and a 1942, you can use the screenmode DblNTSC:HiResNoFlicker, which is basically a scandoubled version of NTSC:HiResLaced. 640x400 resolution, with no flicker. You can only use it for system-friendly apps, though. Since it sounds like you're just using Workbench, that shouldn't be an issue. Both the 600 and 1200 are capable of the Super72 mode, (800x600) but you'll need a monitor capable of displaying 24KHz, and very few are.
Make sure the monitor comes with a 23->15pin VGA adaptor, otherwise you won't be able to hook it up to the Amiga.
Most Amiga dealers still sell hard drive mounting kits for the A1200. If you really wanted to, you could just swap the drive from the 600 to the 1200.
