The D90 is a great camera that is capable of making very nice pictures. If it's in mechanically good condition, with 20k shutter actuations, I think you did well price-wise.

All I could suggest to check for full working order is to take pictures in the various modes. Test the manual modes, aperture-priority, shutter-priority, full auto, program modes, etc. If all work well, then I guess the camera is OK. You could also check for obvious dust bunnies on the sensor. Shoot a clean, blank white wall at small aperture, say F/16 or such. Don't worry about achieving proper focus on the wall. Then view the picture at 100% view on a monitor and see if there are any spots showing up in the image. Dust bunnies are easy to get rid of - there are many posts on this forum about sensor cleaning.

I can't speak for the lens, but I would guess that particular lens is probably one of the better lenses so you should get good images from it. Look for reviews on the lens on the internet.

Regards

My Nikonians gallery