IIRC the Airport Express will only talk to a printer over the USB port, it won't serve disk plugged in -- one would need an Extreme (or maybe a Time Capsule) for that. Time Capsule is a neat idea but the price is ludicrous.
My AE was a wedding registry gift, intended just for iTunes use.
Re: the HP printer, I turned my back on HP after the D145 and the PSC2510:
a) Combined color ink cartridges were a scam
b) OSX software was crappy and flaky as hell
Maybe they've changed in the last couple of years, but I'm skeptical.
