This. I started learning Reason a few years ago, I now do sound design for the Cambridge Footlights
The great thing about Reason is that it's really simple to pick up, but offers pretty much limitless possibilities once you're good with it. Many people complain about the lack of VST support, but it does encourage you to learn yourself some synthesis, and the fact that it looks like a rackful of gear is pretty damn helpful with regards to understanding signal flow and things like that. And with the advent of Record you can now record live audio into it as well, if that's what you're after. My only problem with it is the lack of MIDI out, but I think Record can do that as well (I don't use it myself, someone confirm?).
None. But this doesn't mean you shouldn't look into other hardware. Ideally a computer should be the nucleus of your setup, around which everything else interacts. You can't beat the sheer visceral pleasure of dicking about with some proper synths, and hearing your mix coming together with everything working as one is pretty amazing. That said, it is possible (and often preferable) to do a mix entirely on a computer, especially given how expensive hardware can be. My advice: start with just a computer and Reason, get used to writing your own original music first, learn how synthesis works, learn how MIDI works, start getting some cheap second-hand hardware, get Record or another DAW you can ReWire Reason into and go from there. Most importantly, have fun with it! It is art after all