Don't buy from butterfly labs!
They almost never keep their delivery time promise (google it for yourself).
On their website, it says (in Order Terms)
Bitforce SC (ASIC) products are shipped according to placement in the order queue, and delivery may take 2 months or more after order.
So by the time you get your hardware, 50 GH/s will be worthless.
You'll mine about 0.5 btc in total (as in, in a year although most will be mined in the first few months).
To answer your other question, you would just need a regular PC.
On their website, they have programs, drivers, etc and everything you need to start mining.