I just brought a Nikon D90 with VR kit. How do check if the product I brought is original?

Yes, it has a warranty but not honered in usa

It may seem obvious, but you need a T-mount adapter for Nikon. Two potential sources are Adorama and B&H.

That depends on what YOU consider "best". Do you want the sharpest? The fastest? The lightest? The least expensive? Only you can decide which one best suits your needs.
I have a D90 and the 70-300mm VR lens. I consider this the best for my needs but it may not be the best for yours. You might want the lighter and less expensive DX 55-300mm VR, for example. Or you might want the 80-400mm for its greater zoom range. Or you might want the faster 200-400mm.
Again, only YOU can decide what's best for you.

This is a common problem with this camera/lens conbo. I "solved" it by removing the lens, then reinstalling it - but hold the lens release button down, and rotate the lens a nuber of times (in the camera body) before releasing the button. It appears that the issue is poor electrical contact between the lens and camera body. If there is a poor contact, the camera will not know if thereis a focus and will lock the shutter release. With this poos contact, the camera will only fucntion in manual mode

Hope that this helps

If it's the VR lens:
http://uscamera.com/jaa79851-b37.htm

If it's not the VR lens:
http://uscamera.com/jaa79351-f.htm

As to whether you can replace it yourself, can you use a screwdriver and a soldering iron?

Try a diff lens on the D90 to see if its the body or the lens. Tony

According to and directly from Nikon:

When you see Err on the control panel, LCD monitor or viewfinder of your digital camera it refers to a camera malfunction. Try the following:

1. Remove all accessories from the camera.
2. Install a freshly charged battery into the camera.
3. If your Nikon camera offers resets such as the two-button reset or a hardware reset please follow those instructions provided in the product manual.

If these steps do not resolve the Err message we recommend having a Nikon Authorized Repair Technician evaluate the Nikon camera. Click here for repair information.

Nikon Imaging Products Support Page:

http://imaging.nikon.com/products/imaging/support/index.htm

I'm another very unhappy victim of the defective manufacturing and/or materials in this lens. My 18-105 DX VR lens lived in a padded camera bag when it wasn't on the camera. It's never been dropped, banged, dinged, or put under any stress. 10 months into ownership, with 50 months to go on the 5 year extended warranty, it simply came off in my hand one day while zooming. I was horrified to see a thin black plastic sliver inside the camera body -- the mount just broke off. Thankfully did not get behind the mirror or on the sensor. Greatest insult came from Nikon repair, who refused to honor the warranty because "the lens had been dropped" and asked me to pay nearly 1/3 the original cost of the lens for repairs. When I challenged this and pointed out the as-new condition of the lens, not even a scratch, they simply stopped responding to my emails and phone calls.

There is no digital zoom in DSLR