If you are starting out from scratch with ArcGIS Enterprise, which it seems you are, it will be a sizeable time and financial commitment to get certified in ArcGIS Enterprise. If you don't work for a company/organization that already has access to ArcGIS Enterprise licensing, then an ArcGIS for Developers subscription is probably the least expensive way to get access to all of the various components that make up ArcGIS Enterprise. That said, the minimum Developer plan that includes ArcGIS Enterprise is Professional at $1,800/yr.
