Hi Jake,
I started in the neighborhood of 400lbs about 2 years ago. I'm down to 305 now. I guess I started biking at about 365. I bought a Diamondback Wildewood Deluxe from my local sporting goods store. I'm thinking it was about $300. In any case its a mountain bike. Front fork suspension and seat suspension, which saves ones butt on rough surfaces.
I'd recommend starting with a mountain or commuter type bike. Try to get one where the handlebars can be tilled up and down. I started by raising mine all the way up to give me as much of an upright position as I could get. Also look for wheels with lots of spokes...at least 32.
You will probably wack out wheels pretty quickly, I did. I bought the stuff and trued them myself. If you have a good LBS, then use them. If you are a do-it-yourself kind-of-guy then get what you need and do it.
Read what you can find on the net about bike fit. Make sure you get one that's big enough for you. My Wildewood is about one size too small for me. I had the seat all the way up.
I graduated this summer to a cyclocross bike, which basically a road bike with wider tires. As the gut decreases in size, you will find leaning over not as much of a problem. The curved handlebars of road bikes give you more hand positions which helps with hand numbness.
Good luck.
John