That's great! You even found the correct keyboard for the M24, nice!
There are Windows 1.x drivers for the 640x400x2 video mode, but I'm not sure if they extended that support to Windows 3.x or not. Windows 3 is not the correct version for that system as you'd need an EMS board to make it usuable beyond simple things.
Unless you have a Display Enhancement Board (DEB), the 640x400 mode is not in color.
If your GW-BASIC programs used SCREEN 100 for the 640x400 mode, make sure you're running the M24- or 6300-specific version of BASIC each computer came with. Regular generic GW-BASIC doesn't support the special 640x400 mode.