General question ?

Are you thinking about adding more Nikon speedlights or getting into studio lighting? If the latter, I always suggest starting with one light and a reflector. What do you want to shoot with the multiple lights? Give us some more info and we will be able to advise.

 

Then I recommend:

1 Alien Bee 800 strobe
2 heavy duty 13' light stands
1 large foldable softbox
1 5-in-1 round reflector with bracket for light stand
1 pair background supports with telescoping crossbar
1 muslin background of your choice (not white unless you want to get more lights)

This is a system that you can build on as your expertise and confidence grows.

 

I would start with 42" or so.

 

Nothing is "required", but a meter is very helpful, especially when working with flash. It's a great timesaver. I keep a flash/ambient meter handy and use it for both.

 

Not a must - you can always check your LCD and histogram and adjust flash power etc. until you nail it. However, a meter makes it a whole lot quicker and easier. It also makes it a snap if you want to get a specific ratio between your lights. So . . . I would recommend one.