I'm pretty new to the fediverse, and I find the idea amazing. But one thing concerns me though. How will server owners be able to afford to run servers with massive amounts of data coming through them? Theoretically speaking, if a Reddit migration were to happen how would server upkeep costs look like?
As others have said, the DLCs aren't strictly necessary, but it's good to have them. If you like playing modded, vanilla still has plenty of content. If you do decide to get a DLC I'd recommend Ideology, because it's the most meta-changing DLC.