For the Amigas you can google up some Scart cables, and then some VGA-to-Scart cables. Comparing the two should give you a fairly good idea of how to do it. You want the analog video lines not the digital ones, and the monitor will need to handle composite sync.
The C64 is S-video or composite, yes. The D9800 can handle 31kHz input, is that right? so any S-video-to-VGA converter should work.