There have been some rather painful threads regarding Mac vs. PC.
In those threads it is worth noting that VSL employees repeatedly state that their products work better on PC. Certainly if one is absolutely a Mac devotee, you will do fine. Nevertheless, if VSL employees suggest PC's, that is worth strong consideration when purchasing VSL software.
One key question: what are you intending to create? If you are working primarily with small chamber type ensembles in a stereo mix, less computer power is needed than if you require say 80 tracks in 5.1 surround. My older MIR SE runs decently enough on an older i7 930, but using all 32 tracks pushes its limits. I would not even begin to attempt to run MIR Pro on that sort of system. For what I am doing, a future upgrade to a 3930 would be fine (not happening for some time though).
Depending on your needs, you may very well only need one slave, and therefore only one MIR Pro license.
Finally, note that strict devotion to one particular computer platform is generally not helpful or the most wise way to go. Each platform has its uses, and wisdom is often to use the strengths of each platform (for many: Mac for sequencing, PC for slave). Disclaimer: although I have used Macs in the past, I currently use only PC's.