Mediator won't be able to display native Amiga displays so you'll need a different monitor/tv for that or use the solution I mentioned earlier. You could also use a monitor switcher but you need to switch between displays manually.
As for Zorro graphics cards only the PicassoIV and the Cybervision64/3D have a scandoubler option that will allow you to see the display on a VGA monitor. Some other cards (like Piccolo SD64) have a monitor passthrough and will switch automatically between native Amiga screens and graphics card screens but you need a seperate scandoubler for that to work on VGA monitors.
One of the best cards is the PicassoIV IMHO, but it's expensive.
