Seventy pounds is obviously a very low weight for an adult; unless your height is well-below 5 feet tall, I'm very concerned about your nutritional status. I'd be very concerned about the results of breast augmentation at your current weight. First, given that you are probably quite malnourished, the possibility of significant complications is much greater. Second, from a strictly aesthetic perspective, your lack of tissue support makes you prone to complications such as implant malposition or migration, and possibly even capsular contracture.
Before you seek consultation with a plastic surgeon, your best bet is to see your medical doctor. He or she can assess your nutritional status and help optimize it prior to even considering surgery.
Best of luck.