Weight gain during pregnancy

I am 5'5" and about 172 pounds now at 6 weeks. I am overweight and have been consistently gaining weight in the last 2-3 months specially after conception. 
1) What would be the ideal weight gain for me for a healthy pregnancy. 2)  What would the weight of the placenta n the baby account for at the end approximately?
3) And does breast feeding help with weight loss?