^ Yeah, for a 2x extender you're going to want to start with fast glass - f/2.8 or faster.
Slow autofocus will become a problem too - for example non 1-series Canon bodies won't even autofocus past f/5.6 (there are sometimes tape-the-pin hacks that bypass this).
You haven't mentioned what mount you're after but there is also the Canon 100-400mm f/5.6 L, Canon 400mm f/5.6 L prime or Nikon 80-400mm f/4.5-5.6 if you're prepared to go a bit shorter than 500mm.