Lens:
Yeah some lenses are just worse than others.. my 18-70 for example is awesome (when there is light) whereas my 105mm macro lens is actually quite bad - so normally i just manual focus.. that might be something you could try? Also I think the getting a nifty fifty is an awesome idea - would highly suggest this before buying a flash..
Flash:
I'd only suggest getting a flash if you are serious about photography or do a lot of indoor photography (or have money you have to spend)!
They are called SB-XXX, where the higher the number the better the flash: ie SB-400, SB-600, SB-700 (think this one is new as I didn't actually know about it until I just checked DDP), SB-900 (there is to be a SB-800 but I think the 900 replaced it)
Blur vs ISO
Yes know exactly what you mean - I was taking some photo's at work last week and had the same issue (as I didn't want to use the flash) - in this case you have to realise that noisy photo's (high ISO) is always better than a blurry photo. Also the auto iso can be very useful here.. turn it on.. and set the max ISO you wanna go up to eg 1600 and the min shutter speed ie 1/125 or whatever - open the aperture right up and then go from there..