I don't think that it needs to be mentioned here, but I might as well make it obvious that if you want to "Learn About Computers" then you should probably have at least one computer that is capable of doing what you need. The nice part about this series of topics is that there isn't anything that is particularly strenuous or critical that requires middle to top performing hardware. Pretty much any small laptop or desktop will do. Of course your experience will change based on what you have available.
Obviously the faster the better. The more responsive and powerful the processor is, the better your user experience will be. Keep in mind, we're only one user and we fully accept that if we have to wait ten seconds for the most complicated function to return data, it's quite within our waiting time.
The biggest boost will be in available memory (RAM). The more memory you have the more things will be able to stay in memory concurrently. This will become very evident when you start to run multiple concurrent virtual servers. However, whenever you get to that point, you'll understand how to tweak your configurations to maximize performance.
Hard drive speed and storage will be less critical as nothing we do will consume large amounts of long term storage. There should be plenty of available disk space to do any of these assignments. If your system is on the low side of storage, you will be able to reduce the size of virtual servers' footprints. If you have a lot of space, you can afford to be a little more wasteful, but there isn't much need to do that. The nice part is disk space can be expanded and (sometimes) contracted as necessary. Starting small and expanding or re-sizing available space is better than having to rebuild a new server with lesser but more meaningful storage.
If you only have the option of having one computer, I recommend getting a laptop. Current prices are competitive and you can probably pick up one cheap (or free) from a friend or from a computer store's open box special. Again, there's nothing that we will be doing which will require a massive desktop or power hungry laptop. Most of the time the computer will be waiting for you to issue a command.
Another nice feature of the laptop is the portability. You are no longer restricted to a certain physical location because you can take your computer anywhere and study at any time.