I got up to 312 a while ago due to thyroid issues. The riding did basically nothing for weight loss just so you know, and not much for blood pressure, but it did help my cardio abilities get better more like when I was 20.
The weak spot is the wheels and most bikes have terrible wheels anyway, so unless you spend a large amount there is no difference in what you get. At 400 lbs you would probably need special wheels but at 320 if you get an old mountain bike with 26 inch wheels and 1.95-2.1 range tires then you should be fine so long as you are careful about not jumping off curves. Now I have lost over 80 pounds but I did not really lose weight or get my blood pressure finally under control until I started fasting. That also improved my thyroid and a lot of other issues.