Hmmm, lots to consider. Does the old rig still work? I remember 7.4 being one of the most stable versions ever
Re using a laptop, do you REALLY need it to be portable? I ask because you can get a lot more performance-for-the-money with a desktop machine.
The ProjectMix might actually still be a viable interface on a new system, but you will need to shop wisely to find a computer with firewire, or Thunderbolt(which can work with an adapter cable).
I am hardly the last word on this stuff, but you might consider a recent(not buying new can save you plenty of cash) iMac 27" with an i7 quad and 16-32GB of RAM. Pretty sure they all have dual Thunderbolt so you can connect the PM and a Thunderbolt drive(given the age of the old system, I would copy your old sessions from the OWC to a new recording drive, leaving the old drive as a backup).
If you are set on Apple only, that's cool. But you can buy/build a PC(or a hackintosh) that will kill the old tower and you might be shocked at the savings.