If you can't find an Airport Express on the cheap, try a Neet Airstream. It omits the network-extender capabilities of the AEX (which you wouldn't be using anyway) and offers Airplay functionality that's at least as good as the AEX for half the price.
As for streaming apps, I use FileBrowser Pro (£1.99 I think, may have been £4.99 tops) and Foobar 2000. FileBrowser Pro's only niggle which they still haven't addressed is it doesn't do gapless playback, even in Album mode. Foobar does, but I prefer the interface of FileBrowser so unless I'm playing an album which needs gapless I tend to use FB Pro more than Foobar. I'm sure you'll get other suggestions too.