Reply to First gig and sound quality

I'm having my first DJ gig in march, and will be playing for about 350 people (really excited!) I have no worries about the DJing side of the set, but the amount of songs I need, and the sound quality of these. I will be playing two evening s, with a 6 hour set each evening , which requires a huge amount of songs. My main concern is how to get the songs, knowing that the sound quality is good enough to be played in a club-setting. Is the best way to get high-quality songs to buy them off ITunes? And if so, is it a huge difference between mp3 and wav?