You'll probably be okay without buying a new crank. My advice is to just go ahead and try it and see how it works. Definitely get a chain that matches the number of speeds on the cassette though. With the crank, you might have issues with the distance between chainrings, but this is theoretical. Try it, see how it works. The worse that happens is you have to be a bit careful on downshifts from the big ring to the little; the chain might skate between the chainrings for a moment. As grolby stated though, it's not really much of an issue in practice.

One thing about the crank though, I think Sora uses steel chainrings. If you do replace the crankset or chainrings, the new chainrings will likely be aluminum and thus a fair amount lighter.