Few thoughts:
While a 16 bit TIFF processed out will be enormous, the raw files are really not that "huge" (40-80mb each depending on the compression used). Tethering speed (frames per second, frames per minute, capture-to-onscreen-review time are all very impressive even with average mac laptops. While I wouldn't suggest you use an old laptop, any Mac laptop with 8gb of ram or higher and an SSD will make a fine tethering machine. As you say USB3 is not yet enabled (painful I know) but the Thunderbolt-to-firewire adapter from Apple is shipping this week and we (Digital Transitions) expect to test it on Monday; if it works as advertised it will cover the newer laptops which have thunderbolt, but no firewire, until such a time as USB3 is available.
If budget and weight are no object then a souped up MacBookPro with two high quality SSD drives (one boot, one capture) is the way to go for a tethered station. The graphics card in the MacBookPro is useful to Capture One (used for tethering a Phase One back) while making adjustments.
But the computer that requires the horsepower is the mass-editing/processing/retouching computer. A souped up MBP, top-line iMac, or Mac Pro with a reference-grade monitor (e.g. Eizo) is highly suggested for that where financially possible. Processing out 480mb 16 bit TIFFs, layering and retouching, requires a lot of ram, CPU power, and GPU power, and hard drive read/write speed.
The resolution of the IQ180 is higher than the IQ160, but I think it's important to keep perspective - the resolution of the IQ160 is really stunning. For fashion I think the advantage of the speed of shooting (both frames per minute, and shot to review time) on the IQ160 might be more useful than the resolution gain of 80mp vs 60mp. However, this is highly dependent on your shooting style and print-quality needs. One way I like to put things in perspective is to say that if you capture a sharp IQ160 file and print it well, it will likely be the most detailed image nearly any consumer is likely to have ever seen in person. However, an IQ180 will beat it, so if absolute print quality is a very high priority, and a modest drop in capture speed/responsiveness is an acceptable trade off then the IQ180 it's the only way to go.
You may also wish to look at the Mamiya Leaf Credo 60 and Credo 80 which share a lot in common with the IQ180 and IQ160 (same sensors, same general architecture) but have different color responses and a moderately different user interface. For fashion oriented shooting it's worth your time to visit a dealer that has both (selfish suggestion as we fit the bill) and compare the look of the resulting images from both to see which fits your aesthetics best. Leaf has historically been very well respected for it's color profiles when it comes to skin tone color and gradations, and from our experience so far the Credo continues this tradition.
Re: focus... it's not Nikon or Canon. But it works. Once you get used to the system. I speak here both as a product technical expert and as a user. Would I take IQ180 quality with Nikon D4 focusing? Absolutely, but I can get reliably good focus results with minimal hassle from the DF provided half-decent illumination (e.g. not focusing in the dark) and reasonably static shots (e.g. working with a model moving on a standard studio cyc). This is very personal though (your eye sight, your reaction speed, your manner of shooting) so do not take anyone else's advice on focusing/shooting-speed, get to a dealer and evaluate it in as close-to-your-actual-use-as-possible situations. If you can't get to a dealer most (including us) offer rentals that can be shipped anywhere which count towards purchase if you decide to pull the trigger.