If you want to mine BTC, GPUs are no longer profitable. However, you can mine scrypt based coins and convert them to BTC.
2. You'll definitely need at least 1300w PSU, but 1500w is probably better and will give you more overhead.
4. You'll probably want to get riser cables, otherwise those cards will be pushing out hot air directly on to the other cards heating them up before any work is even done.
5. Any hard drive is fine, a lot of people run off of a flash drive. SMOS-Linux (
http://www.smos-linux.org/) is a decent place to start
6. 4gb of RAM is fine
7. This case looks like it has a lot of air flow, but you could save yourself $100 by just going with a plastic milk crate and a cheap 20" box fan. You might buy it and find that without riser cables and with everything trapped in a metal cage it runs too hot. Then you're stuck with a case you can't use and have to sell to someone at a discount. There are also aluminum framed cases that might work well for you (example
https://cryptothrift.com/auctions/miner-assembly/custom-litecoin-rack-case/).
Hope this helps, I'm new to this as well and this is what I've learned so far from experience/others.
EDIT: It is sometimes cheaper to get 2 750w PSUs than 1 1500w PSU.