Audio Server Newbie...Does This Make Sense?

I am looking at setting up a decent sounding audio server-based system. My existing equipment is as follows: Music Hall CD25.2, Rotel 1068 pre-amp, Rotel 1075 amp, Paradigm Studio 60 Version 3. Please note that I am not a computer expert by any means and I am trying to keep things as simple as possible while doing justice to my system.

I am going to purchase a MacBook and Airport Express "router", which I will connect to a DAC and then to my Rotel pre-amp. I will also purchase an external hard drive for back up purposes.

Questions:
1. Is there anything that I am missing from my set up?
2. If I use Apple Lossless, approximately how many songs can I store per 10G of hard drive space.
3. Is there a special kind of DAC that I need?
4. Other comments.