Making a change to a healthier lifestyle is never too late and there is always a way to achieve that goal no matter what your health issues are.
As frenchhen3 says if your doctor clears you and says it's ok then go for it. So you be proud of yourself for wanting to make this change.
My recommendation is go for low impact and functional exercises combined with healthy eating. Talk to your doctor about what they recommend for you and keep us updated with your progress, we will all be routing for you.
Restarted at 15st 6lb January 2010
Target Weight is 9st 10lb for 13/04/13 49lb still to go
Now at 13st 3lb 31lbs lost so far