If you're not planning on a lot of hills and long distances, you might want to try a beach cruiser to get started--I found big tires were best when I was 320+ pounds.. I'm inferring from your post that you've had issues with activity due to your knees, so you might see some pretty good results just from minimal riding to start with. I wouldn't worry about "selling yourself short" here because you really won't be able to pick a bike with any confidence that it will meet your needs in a year or two. I don't think you'll do yourself any favors by over-investing in a quality bike you might hate in a few months just because it's now too something for your needs (that could go in a lot of ways).
From someone who's been there (I had very bad feet before I lost almost half of my body weight), I wish you well. My basic advice is you've got to figure out what works for you, and if it's working, don't let anyone convince you you're doing it wrong.