Whatever you do, dont get Opalvision as it's not a Workbench supported card, its more like a video (pal,NSTC) video painting system, itegrated like the video toaster.
Also stay away from the original retina1 card as it's not supported by Cybergraphics. Retina Z3 is better...Had the oroginal retina for my A2000 back inthe day only to sell it for a EGS spectrum, then CGFX64, and finally CGFXPPC.
I still have the Retina Z3 s-video encoder somewhere...
The Cybervison with the scan doubler attached was great. Best solution out there. I also had an A2320 A2000 flicker fixer in an A4000, that worked too, but the AGA only displayed 12 bit color when using the Flicker fixer card.
The Picasso line of cards from Village Tronic is nice but the fetch a pretty big price tag.
Ask around here, sure someone in the UK/Western Europe will have a spare card they can sell for the right price.