1. Whats the best system? I am completely out of touch with whats what at the moment, i.e. Final Scratch, Serato Scratch, Torque etc....
serato scratch seems to be the best and most stable by all accounts.

2. When hooked up, is it easy to switch between MP3's controlled by the timecoded discs and a 'real' record? For example could I take off the timecoded disc thats on the deck thats being cue'd and slap on a tune without messing about with the software or hardware?
yes. its just a case of flipping the switch on your mixer from line to phono.

3. How hard is it to setup in a professional club environment? Would it involve powering down the PA or angering the sound engineer whilst ripping cables out the back of the mixer?
piss easy. just like setting up ableton except you'd need 4 free channels to plug into - 2 phono and 2 line-in

4. Anything else I need to think about?
yes. you should think about buying the "damaged trax" back catalog!!!!! :lol: