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.