Are you still looking for help with this project? I have quite some experience integrating Webflow-made websites into custom-built CMS solutions (using Node/MeteorJS). If you’re still looking for a developer with experience with Webflow, let me know. 
Regarding your question about using collections in conjunction with outside databases, this is unfortunately not possible because Webflow does not yet provide an API for applications to communicate with their CMS. However the Webflow founders have mentioned this is a high-priority on their list.