I had eaxctly the same problem with a SCART->VGA converter box. I think the Amiga's RGB output is not quite SCART compliant, so some screens just won't work properly with it. If you have a look in my gallery you will find a screen which works perfectly with the Amiga's RGB output and also has a VGA input for Voodoo screenmodes.
Note that you will still need a 15KHz screen when you get your Mediator and Voodoo set up, as 15KHz screens won't display on the Voodoo unless the software works with ModePro.
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moto