A buddy of mine was probably near 500 lbs when he started his weight loss journey. He 'officially' weighed 464 lbs when he joined Weight Watchers but had been losing weight before then. He used diet and exercise, no surgery, to get under 250 lbs in a couple of years.
As far as exercise, he walked at first then started jogging. Next came running. He got up to running half marathons. He also did low impact machines at the YMCA.
He bought a bike when he got under 300 lbs. He bought a hybrid with only two changes: a 36-spoke rear wheel and a solid seatpost. I think that the wheel was overkill and 32-spokes would have been enough. The original seatpost was a shock-absorbing, suspension post which failed after a few months.
I'd recommend a hybrid or comfort bike without suspension. It might be hard to find as most such bikes come with suspension. I don't know why except it's good for sales.