Some "interesting" ideas suggested here. For quality image results, mirrors aren't often your best bet.
If you want real reach and have the budget, you won't find a better lens (price and quality) that a nice Nikkor 500/4P. You can pick up a great one for under $2000usd. If that's a little on the high end, the still north of $1000usd is a lighter weight Nikkor 400/3.5 ais. I owned the 500/4P for over 15 years and found it to be a truly superlative optic. I only sold it because my imaging interests changed pretty dramatically. At their current prices, you won't ever lose much value if you decide to sell later. Since both lenses use high end optics, but are mechanically simple and lack electronics, you'll be getting a lot of value for the money, and the lens should remain completely functional for decades to come.