Discourse seems to be pretty exciting and am planning to propose this to our client. They want to start small so am preferably looking for open source version. However API integration is absolutely important to support the required custom features. So does open source version of Discourse support APIs (https://docs.discourse.org/) ?
Yes. The āOpen Sourceā version is the only version. The pricing details you likely saw at discourse.org/buy refer to hosting, not a different product. We use the same open source code for our hosting as you get with our official install.