Hmm, depends really. A flash has a steeper learning curve, so you may be better off getting to grips without one before starting on that, but at the same time why not learn straight away.
The 50mm 1.8 is a great lens to have and opens up a lot of low light stuff and DOF control, but it depends on what you shoot. If you are going to use it, then its a good investment. If its going to sit in a bag looking pretty, then it isnt.