You can stream music from a pad/phone/tablet, but there’s no control over library content, ability to edit tracks, rip CDs, etc.
Bugger cloud content, you need to be able to access your music anytime you want it, and anyone who imagines that you can do that everywhere in this country is deluded. Same with Spotify, I’m not paying £120/year to basically rent music, that money would buy me a lot of physical music that’s actually mine, not crippled by the vagaries of what passes for mobile broadband in the UK.
Look for refurb MacBooks or PowerBooks, check out the specialist Mac magazines, and look at the classifieds; there are often agencies upgrading, or bankrupt stock going for decent prices. I’d be a bit wary of eBay for a top-end laptop.
Thinking about it, the Mini is an ideal little media streamer, so shoving some more RAM in, getting a nice big fast external HDD, and keep all the iTunes stuff on the external, possibly a NAS drive, then just use Remote on an iPhone or iPad. That’s how mine’s set up, then you could add AirPort Extreme’s around the house with active speakers over time.
Just a thought, you keep your Mini, which is working fine, and add flexibility to it.