Nothing at all wrong with it, it's a very good one. I just happen to like Samsung very much. A single platter 320gb is quicker than a double platter 640gb one, but barely even measurably. By far the sensible choice is the WD one
Antec 300 is the standard recommendation for cheap cases with good reason, my best mate has one and is quite happy with it. Cheaper cases tend to have worse internal layout, thinner side panels, and fewer removable panels. They're generally not as nice to work with, things like removable motherboard trays are lovely. Thicker panels and better joining leads to fewer vibrations and a quieter system. The case is one of few things that survives many builds, so I reasoned buy a damned good one and it'll last for years. The Akasa ones certainly feel like they'll survive anything.
Mouse and keyboard is a very personal choice. I have really cheap ones, but have come very close to buying a 50 quid keyboard a few times. They just don't seem like a very sensible place to allocate funds.
Laughing hard at the cosmos question. Yes, I think its worth it. That's not the one I'd choose though. It's important to love your computer, it means you treat it better. Find a review of it and read through it, see what you make of the internals. If you're willing to drop 150 on a case, that opens up a whole world of goodness. Find a review on the akasa omega, it's a work of art.
Here's a 2041 post thread about how awesome the Akasa cases are:
http://forums.overclockers.co.uk/showthread.php?t=17671494
They all use the same frame, the omega has a door, and 120mm fan mountings in the top and side. It's nicely laid out for watercooling as a result.