I am not one to follow the current beta to a "T" so I often try new champions out in new situations, like a janna mid. The thing about janna is that it is her CC and team fight control that make her valuable not her damage. So she really doesn't need the gold or the levels. She also doesn't scale that well in dmg in comparison to other mids.
In terms of sustain, thats on of her problems even as a laning support. Thats why you may see a support janna with heal and voyance instead of Flash and voyance.
My only advice if you did want to mid janna is to either be very good with blocking with your shield, take teleport, and/or start with a sustain item (boots/3hp, regrowth/1hp, blue gem/2hp, meki/2hp) instead of a dorans ring