Moving to itunes as a server and music playback help

I have a atv3 and ipad2 and am buying my little one a ipad2 for xmas so im keen on buying a 2tb network harddrive sticking on my router and using my accer windows7 laptop upstairs on homeplugs to see the itunes and just download the apps films music and sync ipads. But i have found that when i connect it to my dacmagic and use itunes as music player as well as a server the artwork lags at times i get not responding and it seems slow yet its only using 1/4 of gig memory.

So my plans are to buy a second hand mini mac and leave usb connected to my dacmagic 24/7 and vga to my tv for the odd time i need to browse itunes plus control itunes via the ipad for music and home sharing to ipads and atv, i have 122gb of music 34gb of films (which i will be adding to now im buying off itunes). 7 gb photos so whats the oldest cheapest mini mac that will see my itunes stuff of the network drive and be able to do what im after without lag. The only other programs i would run 24/7 on the mini are garageband for vinyl copying ,spofity and dropbox for live photosyncing of my phones. 

I have looked at loads and theybrange from £90-700 so i was hoping £150-175 to start me off and then if it goes well and itunes is awesome on the mini mac i would would save and buy a new one from apple store. 

Thanks inadvance for any help given simon