Hello,
The two options I can think of off the top of my head are:
1. Apple TV: No need for an external computer for playback, can be connected to an external DAC, and can be controlled via an iPhone. The downside is that you are limited to the size of the hard drive on the ATV.
2. Sonos: No need for an external computer for playback, can be connected to an external DAC, allows use of NAS for storage, and can be controlled via an iPhone. Downside here is the cost for both the Sonos and external NAS can get expensive.
All other options require the use of a computer (Transporter, Squeezbox, AirTunes, etc), and may not provide control via an iPhone. The simplest and most cost-effective solution I can think of is running an Airport Express via AirTunes using it's optical output: total cost is $100 and the device is the size of a deck of cards. You will, however, need a computer running iTunes for playback.