One focusing tip is to use the AF assist illumination (the bright light on the front that flashes, if yours is turned off you can turn it on through the menu), assuming that it's not too bright, or even bring along a flashlight with you. That should help the camera focus nicely. If that fails, a good trick is to use live view. A few months back I shot a dinner as well, the lighting was terrible! Our table's light was actually burnt out, so lighting was a challenge. I actually ended up mounting on my old 50mm f/2 AI-S lens and manual focusing with Live View, assuming that you can keep the camera steady enough, you can even use the zoom feature to get an even more precise focus.
Regarding flash, it's always good to invest in one. The pop-up flash on your D7000 gives a disgusting, harsh, reflective light that's very unappealing. When you invest in a flash, you get much more power to light more, you can bounce the flash off walls and ceilings for a more natural light, or even buy a mini softbox to soften the flash's light. You should take a look at Nikon's new SB-700, an excellent high-quality flash, or if you're on a budget you could probably pick up a second hand SB-600 anywhere from $180-$220.
By the way, you don't need more lenses, you WANT more lenses. Of course, there's no such thing as too many lenses!