It depends on how many players your server will have (and how much they build). Minimum requirements (for a small server, i.e. only 1-2 players) would be a single core CPU and 1 GB of RAM, however, it's highly recommended to have at least a dual core or better a quad core CPU. For big servers with several players, 4-8 GB of RAM should be sufficient
Having an SSD is also helpful, especially when using SQLite, but of course it's not required.
Make sure the server has a good bandwidth since this game causes a lot of traffic (world sync, custom images etc).