Seems to me a 600cc sport bike is a little big for ya unless you've been riding your whole life. Seriously, I'd opt for a quasi sport bike, or cruiser (maybe a Triumph Bonnie) to start with. I have a nighthawk 650 sitting in the garage you can have for the downpayment of $800 and nothing more to pay. Oh yeah, it's got the same horsepower as a BRAT does so it definately isn't a slouch.
Definately take a safety course. Trust me, it isn't you it is every other idiot out there!!! Never, EVER, EVER, get insurance that isn't stacked full tort for a cycle!
As for effiency though, you're much better off buying yourself a Toyota Echo with 80k on the odo for the same price then you are going with a bike.