If you're on a tight budget a past-generation model is the most bang for your buck, perhaps the 400mm f/5.6 or an older 300mm f/2.8 plus a teleconverter. I had

If you're on a tight budget a past-generation model is the most bang for your buck, perhaps the 400mm f/5.6 or an older 300mm f/2.8 plus a teleconverter. I had a 100-400 for a while and I wound up shooting exclusively at 400mm, so it was kind of a waste of weight & cost. Also, glass is more important than body, but you might be able to get more stops for your money with an updated body, e.g. if your current body is only good to 400 or 800 ISO. IMO on a budget you should get the best f/5.6 setup you can and then look for a body with high ISO.