You can expect the 645z RAW files to be in the 60 to 80mb range. If you only use LR (or Camera Raw) for adjustments to the picture and cropping, then you don't need much computing power. When I travel I even use my Surface Pro on occasion. A bit sluggish, but manageable.
As soon as you start doing composites or focus stacking, or brush or gradient filters, well then you will find a while paradigm shift is needed in computing power.
Using Photoshop with a lot of editing, or adding Camera Raw smart filters, or adding multiple layers with masks, or blending.... Same shift in power needed.
The biggest adjustment you will need to make is when you discover how much content exists in the 645's raw files. A blown out sky will reveal unbelievable details when a masked gradient filter is applied. Processing power is the name of the game to reach the full potential of what the 645z can give you. The only area the 645z is unforgiving, is focus. For the rest, you have so much lee way to be sloppy, or just make mistakes which you can fix in post to end up with amazing pictures.
I found that more RAM is more important than a fast hard drive, and fast GPU is just as important as a fast multi core CPU. I'm not a MAC guy and can't comment on the capabilities of the MAC vs Windows, but earlier this year I was seriously looking for more proceeding power and was considering a MAC, if it could deliver.
I eventually settled on the HP G5 zbook with xeon CPU and quadro GPU. I selected the 4k version with the color corrected Dreamcolor screen. Cost almost as much as the camera, and I found the following....
For normal adjustments in LR or Camera Raw, I did not see much difference with my Surface Pro. Until I started to load 20 pictures at a time in Camera Raw, added brush or gradient filters with masks, or composites, or Photoshop with multiple layers. The HP does not slow down. The CPU automatically goes into overclocking beyond 4ghz, the cooling fan sounds like a 747 preparing for takeoff, but the speed is unbelievable.
Having said that, it took me a while to figure out the settings needed to achieve that. Between LR, Camera Raw and Photoshop are distinct differences that makes it faster or slower. Took me a couple of days of trial and error to discover those, and is very happy now.
Have no idea what I'm going to do when the FF 645 comes out, because then the file sizes will likely go from 60-80 up to 200mb. Will cross that bridge when I get there. Need lots more money.....
Have fun with your 645z, it is an absolutely fantastic camera, I've had mine now for abt four years and are still discovering new tricks with it.