Hi Karl,
The Yamaha A5000 is a real creative workhorse and will fit the bill for what you want to do. It ships with some CD's containing synth, drum and instrument samples, the internal effects are fantastic and it has some really nice sample editing tools that are better then Recycle etc. Plus its hardware so there's no latency and the sound quality is booyacka. (Makes me sad to write this in a way... as it's so good.. but I've just not used it enough in the last three years..)
Midi wise you just whip a midi cable in/out from your M-Box or midi controller and off you go. It's also multi channel on midi and audio so you can do more than one thing at once. Playing it with a midi keyboard is brilliant as all the parameters are velocity sensitive (e.g. the harder/softer you hit the keys you can change volume, attack envelope, filters etc.).
Make me an offer circa £300 plus P&P if you're interested - it's boxed in its original box, plus all disks manuals etc and it has the maximum memory and the internal hard drive for sampling - drop me a pm via the board.
Ben
PS _ The Supernova is basically an analogue modelling synth and will do anything you want synth wise. It's drum capabilities will only stretch to 808/909 (synth drum) type sounds though 'cos it's not a sampler. It's a fantastic 303 emulator too 'cos the filters are totally badass.