You need 1 battle.net account, which is free but includes no games outright, then you add 5 wow accounts ($5 each at the store now)

No discounts for multiple accounts, unless you feel like prepaying for up to 6 months at a time, you could theoretically stagger the 6 month subscriptions and pay a few bucks less a month per account, but that's a bit of a stretch for most people to assume that they will still play 6+ months into the future.

Anything is possible, some encounters may require more gear and more creativity, but it can usually be done. I remember we had a 10 boxer boxing 10 man ICC back in Wrath and he managed to do the lootship by himself, before it was nerfed.