I'm not a DJ so I have no comment on that M-Audio rig, but I do a lot of programming/performing of live computer music and I think you may want to check out MaxMSP + Ms. Pinky (
http://www.cycling74.com/products/mspinky). Out of the box, Ms. Pinky lets you use vinyl records with time code information to control audio (and/or video) scratching on your computer. If you already have a pair of turntables and a mixer you will be able to do a lot more cool stuff for the money with MaxMSP + Ms. Pinky.
MaxMSP is a visual programing environment that if you master it, lets you create pretty much any live computer music software you can dream up. If you don't feel like teaching yourself MaxMSP, Ms. Pinky also comes with some premade Max patches for doing audio/video scratching so you don't need to do any of your own development.
When I said you have all the hardware you need for Ms. Pinky, that excludes a couple of things. When you are talking about doing sound out from a laptop, are you thinking of getting an external audio interface so you can go 1/4" out from your computer vs. the headphone jack? While it is another piece of gear, it will sound better, and you could use some of the money you save on the M-Audio rig if you get MaxMSP + Ms. Pinky instead. Also, you may need to get a second phono amp (I've seen a stereo phono amp with no mixing that is only a couple inches square) because of your signal flow.
[turnables+Ms.Pinky vinyl] -> [phono amp] -> [audiointerface+computer] -> [sound out goes to your DJ mixer, no phono amp needed in this stage]
I don't know if this interests you, but there is a video processing add-on for MaxMSP called Jitter (included free with student copies of MaxMSP) and Ms.Pinky + Jitter would let you get into VJing.
As far as MacBook vs. MacBook Pro it depends on two things:
1. do you want to do video DJing? I'd get the Pro if you can afford it if you do, although the new integrated graphics should be an improvement.
2. are you getting an external audio interface firewire is usually a better choice because of the way the protocol works firewire 400 > USB 2.0 for streaming audio or video (to interfaces or to HD). The one downside of firewire interfaces is since they allow for better datatransfer, you usually will get much more ins/outs in a firewire interface and for that reason they usually cost more. If you only ever need stereo in/out, USB 2.0 may be fine if you want to save some money.
Let me know if any of that doesn't make sense.