take my answers with a grain of salt haha I'm still learning
1. I think your camera can communicate with it. I have the D60 and SB600 and I know that for sure needs a trigger. Beyond that I don't know how.
2. Not sure. If you want something cheaper I know many just use the silver things you put in your car to reflect the light and have someone hold it
3. Take a picture of your grey card in the setting that you'll be taking your subjects, then in ACR use your white balance and click on it. That tells it what is supposed to be 18% grey and you can copy those settings.
4. Don't fully understand it but I was reading a tutorial on ILP (probably by Andrea Joki) and she stated that when you take in RAW the camera doesn't use your custom wb settings. So if that is correct then don't worry about wb in camera, take auto and fix in ACR
Ok well I'm sure someone will correct me haha so hopefully I was semi-correct in what I said
I'm pretty sure that you can use your onboard flash to trigger the SB600 remotely. I have the SB800, and I put it on the Remote setting. Then the on board flash will trigger the OCF. I put my on board flash on Commander mode - you'll have to look in your manual to see if the D90 does that!
I recommend getting the fold up 5 in 1 reflector. It works for all kinds of things!!! If you are working outside, stands are tough because the wind can get the better of your if you don't have someone to help assist for you!