It's great low impact exercise!

As far as bikes are concerned, test drive lots and maybe end up with something used to start. If you really enjoy cycling, you'll probably want to upgrade soon.

I'm also overweight. I bought a "comfort" bike about five years ago and rarely used it. I felt like all of my energy was going up and down in the shocks and cushy seat instead of forward.

Two and 1/2 years ago I bought an "urban hybrid" bike - skinny tires, flat bar, low-end road components with a mountain bike style frame. That ended up being a much better bike for me, and I started commuting and doing longer rides of 20-25 miles.

Six months later I was so excited about riding that I bought a carbon road bike. My body has changed greatly, and now I'm thinking about doing my first century.

Good luck with whatever you decide. Just make sure it fits you well. If you get a bad fit, you can get discouraged quickly.