Make, buy, or partner mobile app?

There is an mobile app I want to create. There are a number of apps that offer a good portion of what this app would have but not the key functionality. I could make this from scratch which would be time consuming, I could offer to buy an app from someone else, or I could see if they want to partner. Has anyone acquired an app from someone? Did it work out. Has anyone partnered with a developer and changed that person's app? Thoughts would be appreciated.