I am a D90 User, have no problem with the kit lens and i will only go for Nikkor lens. at 17mm-50mm i dont think you will have problem with low light shots. Get a good flash like SB900 (Turn off the temp protection function) which will cover your range of 18-105m Kit.
I would suggest to go...
I have got D90 Months ago and tried my Friend's D300. D300 is heavier the D90. I rather spend extra money for Flash (SB-600), Good Tripod (Manfrotto 190XPROB+496CR2 or 488CR2) and may be some prime lenses and Cokin Filter system.
My D90 also survive Mount KK right up to the Low's Peak :cool...