Hello! Adding other country-specific Amazon sites seems like a popular request, so I am adding it to the list of new features to develop in the future. Can't give you an ETA yet since I am only working on this site in my spare time, but stay tuned. As to the min# of reviews -- yes, that's an easy change. Would you like me to specifically add 20 as one of the stops along the slider, or would you like to have the ability to key in any arbitrary number (like I did the price slider)?
I added 25 as one of the options for filtering by review count (seemed like a more even increment to go from 10 to 25 to 50).
I also just added a dropdown on the top left of the site that allows you to choose a country-specific Amazon instance. I expect that some features will not work for non-US sites, so check it out and let me know what you find. I may not be able to fix them all due to regional specifics, but if something is broken due to a bug in my code I may be able to make it work. Thanks again for the suggestion!