RWR wrote:
There are a couple of 400 f/5.6 lenses that won’t compromise your image quality and should fall within your budget - Nikkor ED Ai or AiS, and Tokina AT-X AF. I have the latter lens, bought new when it first came out, and have used the Nikkor: little difference in image quality between the two. Check Ebay, B&H, KEH.
I have that 400mm F5.6 Nikkor ED AiS. You really need to be on a tripod for that lens....