Outdoor portrait camera settings?

I'm having an early evening outdoor photo shoot at around 6 pm, assuming it's sunny and probably one hour before sunsets. I want to know the best combination settings;

1. Which shooting mode (A, S, or Manual)
2. Which ISO
3. Which Exposure Compensation.
4. Which White Balance

I have a Nikon D90, 24-70mm F2.8 lens and SB800 speed light (is speed light needed?)

Your professional advice is greatly appreciated!
Best

* Shoot in the shade to avoid harsh shadows but were the warm indirect light from just before sunset lights the model
* Set white balance to shade and bright sun. Shoot some shots using both white balances
* Shoot in RAW
* Use the cameras light meter to determine the correct exposure
* Shooting models or portraits, stop the lens down two stops
* If you do use the SB800, add a 85b filter to match the warmth of the sunset light

Sample of such a shot.

1. Whichever you need
2. Whichever you need
3. Use the light meter to determine the correct exposure
4. Shoot RAW, then it doesn't matter, because you can adjust in post processing
5. If there's not enough light, or if the background is too bright you may need flash

There's no way for us to tell you the settings to use in advance, since we have no way of knowing what the lighting conditions will be at the time of shooting. You need to learn how to make these decisions yourself.

Just make sure you don't shoot in harsh sunlight, find a shaded area. Don't use anything wider than 50mm for portraits. You may need a tripod.