1) PC works juts as well for “getting the most” out of djing, the quality of your performances will not falter based on your platform. The only real justification for macs (imo) is dependent on whether you produce/do graphic design, and whether your workstations are mac platforms. After dabbling with various DAWs, Logic felt like the best fit for me.
2) I would suggest the largest size in which you feel comfortable carrying around with you. I use a 13″ 2010 pro, and it gets the job done, but with Traktor I really wish I had more screen real-estate. A 17″ (which I believe mac is discontinuing) would be ideal IMO, but the 13″ can more than get the job done once you get used to it.