Having a ZX Spectrum would in no way actually help you learn hardware. The only thing it would do is to reinforce its realness, so to speak, which would help to motivate you to learn it. But it would not actually help you learn it – the design and learning curve of assembly language is what it is, and having access to running hardware (compared to having an emulator) does not change that.
It makes it easy to learn the hardware because it has a simple processor and makes it dirt simple to access. Windows on the other hand runs on modern chips and it isn't easy to run assembly without 3rd party software and graphics is also not something a child can easily handle. I suppose a good emulator would be similar, but emulation is a little bit of a lie :)
