Thanks for the "Thumbs Up" on the Forum, I can't take all of the credit though since I do get a lot of encouragement and support from seeing how much time other D.J's put in on here to offer advice and just keep a good atmosphere going
Yes, a lot of people use Mini Disc as a Media for D.J'ing although I wouldn't spend a fortune on a player - a general good quality domestic Sony or Technics will do the job.
I think that MP3 is slowly overtaking it though, and maybe it isn't as popular as it was in the late 90's when they became affordable, but I don't think that Mini Disc will die out altogther it is still also popular for solo artists for backing tracks etc.
Certainly CDR and PC technology is making MP3 accessable to more and more D.J's and this is having some impact on Mini Disc - especially with blank 80 minute CDR's costing as little as 14p each, and around 6 out of 10 of the D.J's that I deal with certainly use MP3's in some capacity, but it is illegal.
Most older D.J's used Mini Disc to replace their old vinyl collections (Again check the legality of this) and maybe this would be an option that you are considering.
Regarding you proposed application, then the cheaper Sp500 based system will be enough to meet this criteria. Of course if you business does grow, you can increase the output slightly by adding another set of speakers, or just upgrading the Amp & Speakers.