If you are only going to use it for throwing punches then a 70 lb bag should be good enough for a beginner, but if you plan on kicking it then you should get a 100 lb bag as it will sway a lot from the kicks.
This one has an awesome discount: http://store.titleboxing.com/twins-profe...
Other options:
http://store.titleboxing.com/canvasheavy...
http://store.titleboxing.com/hbv.html
http://www.ringside.com/ringside/ringsid...
Everlast 100 lb bag and 16 oz gloves
Having a bag isn't gonna help weight loss lol
look up Crossfit
I weigh 230 pound. The main purpose I want one is for weight loss. I would prefer one that hangs off the celling and is fairly cheap but good quality. Thanks.