Sid, if I had to choose between the used D90+lens for 650 vs the Camerabox price of a new D90 body only of 540 I would probably go for the new body and get lenses separately. Plus with the new body from Camerabox they are giving a free 5 year warranty.
Sigma has some nice lenses like the 50mm 1.4 which I love but I don't have a good feeling for the 18-200. It also depends on which version of the Sigma 18-200 lens they are offering used: the earlier version which doesn't have OS (Optical Stabilization) or the newer version which does.
PS: some recommendation of lenses to use with the D90
- ultra wide angle >>> Tokina 11-16 f2.8 ATX (DX lens) the best there is, optically better and cheaper than Nikon's 12-24, but the range is shorter
- On a budget? Nikon 50 f1.8D prime (FX lens), 35 f1.8 G DX lens is also good but slightly more expensive
- Budget portrait lens >>> Nikon 85 f1.8D (FX lens)
The Nikon 50mm and 85mm f1.4 lenses are also great but they are more expensive compared to the f1.8 versions.
(depending on your budget and what you want to do)
- mid range zooms
>>> 16-85 VR DX lens, 18-105 VR DX lens, 18-200 VR DX lens, 17-55 f2.8 DX lens, all good
- longer telephoto zoom >>> 70-300 DX lens
However, if you plan to become even more serious about photography in the future and have a feeling that you may upgrade to 35mm/Full Frame/FX consider your purchases carefully. Also think about re-sale value (where I live Nikon and Canon equipment can be re-sold easily compared to Sigma, Tamron, Tokina, etc).