Unless you know that you'll need the flash, I would wait on it. The ISO performance of the D7k is outstanding. I've just been shooting indoors at 4:30 in the afternoon, with no direct sun, and the shots at ISO 2700 - 3600 look really good (at least on the LCD). In any event, you can always pick up the flash if you miss it.
I shoot primarily with the 18-200. Again what you need depends on what you do. The other lenses offer better apertures, which is very useful (and again mitigates against needing the flash). However, it would mean potentially carrying more gear and weight and changing lenses, which itself is inconvenient.
You might consider starting with the kit lens (18-105) and see how the aperture, quality and range work for you. The quality would be roughly the same as the 18-200, although the range is obviously shorter. It's "only" $300 and you can always get a good portion of it back if you decide to upgrade and sell it.