Keep up the great work! Riding bikes is a great way to lose weight since you can get out and enjoy the fresh air and scenery instead of sitting on a treadmill watching TV.
I would also recommend a used mountain bike with no suspension. Most quality 26" rigid mtn bikes should do. If you need some brands I would try Trek, GT, Diamondback on craigslist. These are in no particular order of course but will provide you with good quality.
Keep us posted with what you choose!