Nikon D600 or D800 for Fashion?

Quick background on me:

I am moving from a Nikon D60 to a combination of either a D800 or D600 with a D7000 as a backup body.  I mostly shoot fashion and portraits.  I don't shoot studio, mostly location and shows (runway). I plan on growing my business and I need to invest in tools that will meet my needs over a period of years.  I have a SB-600 for the D7000 and I will invest in a SB-910 for the D800 or a SB-700 for the D600.  I will use a battery grip on all bodies.

I always shoot in RAW.  Post is done with Nikon NX2.

Glass will be:

Nikon 50mm f/1.4G

Nikon 105 f/2.8 VR

Nikon 70-300 VR

Tamron 24-70 f/2.8 VC

Will the D600's limitation of 1/4000 second shutter speed impact well lit outdoor shots when using lenses wide open?  How much of a real issue is over exposure with this camera?

Will the 1/200 flash sync cause issues with normal runway shots?

Does the D800's resolution give it a noticable image quality jump over the D600 when printing relatively large or will 24MP be more than needed for high quality print work for magazines and poster sized prints?

I appreciate your time and assistance.

Scott