I find the 18 70 lens great with this camera.
If you are going to use the flash off camera go with the SB600 or SB800.
If money is no object go for the SB900.
I find the SB400 great with this camera. It is small and light and cheap!
I have all of the flashes but find I use the SB400 mostly.
In my home studio, studio flashes are not much more expensive than new flashes so consider them too!