Need help creating apple home network

Hello. We recently moved to a new home and have a new computer and want to create a home network. My old set up and new set up are very different and I'm trying to figure out the best way to configure it before I get started so I don't have to re-do it.

My current (old) set up:

MacBook (old white one) running OSX 10.7.5

itunes music files on external HD

time machine backing up to second external HD when I remember to do it

iphoto split between older files on third external HD, current on the macbook bogging to down

We also have an iphone 5, and ipad

We purchased but have not set up an imac with 1T HD, and just recieved an Apple TV for Christmas. The house we moved into has hardwired speakers throughout. We have Comcast cable just set up. Wireless router signal is really bad and not picking up any signal in 3/4 of the house.

My goal is to have the iMac be the hub of the house. Move all itunes and iphoto files there. Clear up our macbook so it runs better. I'm thinking it would be best to have our router be an airport of some sort rather than use comcast for that, also want one dedicated to itunes hooked to the stereo, and then others to bounce the signal throughout the house. Has anyone done this? Should I pay more and get the timecapsule as the router or is it fine to stick with different HD for backup and get an airport express or extreme instead? Do I need the extreme or express? How far should that signal repeat take me? Anything I'm not thinking of? And while I'm here... has anyone used the Comcast modem/router just as a modem? Is it difficult to turn off the router? Does Apple TV repeat the signal or is it just dedicated to it's primary function? Thanks for any advice!!