I'm definitely going to agree with the comment on the gear. However, the safety courses are not a joke, because honestly, an empty parking lot is a lot more of a controlled environment for someone to learn than just being put on a bike on the street ,granted that I was put on a bike on the street and sent my way. It's not the best way of learning because some things you do want proper instruction so you don't have to go through the difficult and often painful process of learning the hard way as much as possible.
And no an experienced rider might not be able to teach you to ride properly, it's like skiing, many people know how to ski very very well, but however, when it comes to teaching, they can't translate their own skills into words, or they just can't explain concepts properly, find an experienced instructor who knows what they're doing.
As for gas mileage, it really depends on the bike you are getting, take the honda cbr 125r for example (yeh I know 125cc sounds like child's play but for a beginner, its enough to learn on) has excellent gas mileage, designed in Europe to meet the stringent environmental standards there. Also the greater the mass of the bike and the rider, the more fuel is going to be used up, if you haven't already figured that one out.