And once you've got one lens, use it for a bit. Work out what it's good at, and work out what it can't do. If it can't do something you want to do, add that requirement to your list of future lens requirements. Then when you've got the money for another lens, see which requirements you could fulfill, and buy a lens to do that (or more likely, wait and save up more!). Don't plan a list of things to buy before you've got used to your current stuff.
Having said that, I would rate a fast-fifty as a very good lens to learn with, once you've worked out where all the controls are on the camera.
You'd be able to shoot at just a smidgen less than ISO 100 in that case, as f/2 is two stops wider than f/4.