Hello @delman3d
I think it’s important to differentiate between useful tools and artistic skill. Yes, Zbrush Core is absolutely useful for the honing of artistic skill, which is a critical skill you will need to get the best results out of Zbrush. Zbrush on the other hand absolutely contains features which would be extremely helpful to anyone working in a production workflow, or looking to have advanced control over a mesh. Those tools will have a lot of value for you, but they dont render anything else completely without value.
So if ZBrush Core is within your reach right now, I would take that as an opportunity to improve your artistic ability, without which all the advanced tools in the world wont make up for. It also has the added benefit of training you in the way the program works on a fundamental level, so at such time as you might be able to upgrade, your learning curve will be drastically reduced.