Clone a Complex SaaS and add a Few Improvements

We'll just be blunt: we have a competitor who has a nice SaaS, but they've missed a few opportunities in their product that we'd like to add to our own. We have our own ideas for changes and improvements, but we need help getting started. We are ideally looking for someone to help lead the project and tell us how to get there and oversee the project along the way. At the moment we're trying to get a ballpark of the estimate initial cost for a minimum viable product to get us started, and then we'd like to keep you around to add more features over time. Please take a look at, and sign up for, a free 7 day trial of [login to view URL] as the backend very closely matches what we are after. I would imagine this will be based on node.js but please provide us with your own thoughts and input as we are very open to ideas. This will be an extensive ongoing project that you'll be involved with, but for now, we'd like to know what we are looking at to get off the ground. If you can take a look at their backend after signing up, and either give us a fixed cost for the whole thing or break down the cost by specific features, those will both be helpful. Our budget isn't large, but we're hoping to start out small and get revenue coming in to support extensive further development as we go. Please include 'possum' in your response so that I know you read over the project description. You don't need to include it in a sentence or anything, just add it to the top of your reply.