Newbie question

Hello there

I'm thinking of purchasing tomorrow a Raspberry Pi for my livingroom and installing XMBC.
Looking at the installation help pages on here, seems pretty straightforward, but would like to just doublecheck and be sure I'm doing this right!

I'm going to purchase a new 3TB external HDD to plug into it.

I should be able to connect and control the XMBC via the iPhone app?

Will XMBC automatically find films I stick on the external drive?
What's the essential apps people recommend?

Also, if I wanted to watch telly in the bedroom, can I set up a secondary Raspberry Pi and XMBC as a 'slave', drawing everything from the 'master' one in the livingroom?

Many thanks for your help guys!