A few newbie questions

Hi, I am relatively inexperienced with digital photography and new to Nikon.

I've been using Olympus for over 30m years, but... I got a Nikon D7000/18-105 kit as a Christmas present, so I have switched.

So far I have added a Nikkor 70-300mm (I'm sure going to miss my fast 2.8 Oly lenses but can't afford high end glass now and I don't shoot as much as I used too).

I am planning to add a used sb-800 or sb-700 flash and a sc-28 or sc-29 cord.

question 1 - will I benefit at all by sc-29 over sc-28 since d7000 body has its own AF assist light?

question 2 - If I do use sc-29 or sb-700 mounted directly on camera, which AF assist light will be activated camera's or sc-29/sb-700 (or both simultaneously)?

#3 - If and when I do decide to upgrade to better lenses... I would love the 17-55/2.8 but that’s way out of my budget.  Would the 24-85/2.8 work OK on a DX body – Nikon site says that it’s FX/35mm format? What would be its limitations on DX camera?

#4 –atalmost double the price,how much better really is 16-85 than 18-105? Comparing them on Nikon website I can see no difference in specs other than size and weight.

That’s it for now ... many more to follow I am sure

Thanks in advance for your input a and advice.