Setting up wireless digital music server

I am interested in putting much of my music collections to digital and control with an iPad. I have an inwall speaker system (and HT and 2 channels). The inwall speaker system would be the main target, but I also would like to access to the digital music library from other systems (plus on my laptop via wireless). Currently I use an integrated tube amp to connect to the inwall speakers.
I have mp3 files stored on a wireless hard disk. I don't have an iPad yet, but I will buy one and try to convert those mp3 files to iTunes library, and use the iPad screen to browse and select music. But I am not sure what other options are available for the rest. I might need to replace the int tube amp to a digital receiver that can connect to the wireless music server?
Any suggestion?