Hi,

there is already a Search API ranges module, which provides a slider widget. Also, importantly, Search API also moved to the Facet API, but this module does not seem prepared for Facet API. See #1213360: Move to Facet API

With the new Facet API, you can easily add widgets to Search API ranges. So please merge this with Search API ranges, by adding a new Facet API widget. I will be happy to apply any patches.

I don't think a new range module is necessary/useful.