A 100-400 that fits into the 500 pound budget together with a body, might be quite worn out. I also do not think a 10 year old semi-pro body from either manufacturer is a good choice, as the sensor is simply old and every entry level DSLR runs circles around them when it comes to image quality. On the Nikon side a D5100 and any successor in the series should do just fine. I got myself a D5600 for holidays and it is really amazing how good this little camera is. Sure the handling is not semi-pro style and you need to use the screen to change some settings that can be changed by pressing a button and turning a dial on the semi-pro bodies, but if you are new to photography the solution with the screen is maybe better, as it also offers you an explanation on what the setting does.

In the end we should not forget that airliners are easy subjects and pose no challenge to any modern ILC camera. They are big, usually have a clear contrast against the background and fly in a predictable...