I would recommend keeping your plans intact, but including to perhaps a echo indigo card (if the 12inch has a pcmcia slot?) or a small usb dac (total bithead, m-audio etc - you may or may not need the separate headamp depending on which you go for). Then you'll have a portable setup which can suffice for home listening for a while.
Then, if you wanna go further down this expensive hobby, save all your pennies and get a killer DAC or CD source for your home setup down the track.
I have all my music stored on my powerbook for both home and portable listening.
At Home I'm setup like this:
Powerbook > iTunes lossless > Airport Express > DAC > amp > speakers/580
When portable, I'm afraid I'm stuck with the powerbook's headphone out for now, as I'm still waiting for the right portable device to present itself. It is quite torturous, however, going from my nice home setup to suffering the sound of the powerbook alone (which used to sound quite good some time ago). That's why I recommend getting a good portable source first, once you've tasted quality audio, you don't wanna go back!
Oh, and expect to fill up that hard disk with music pretty quick!