Go to a good bike shop, there should be some nice bikes for $500. Not great, but nice enough. Especially one that sells Jamis or Fuji, both companies seem to give a bit more for the money than Trek or Specialized. But most brands have bikes around $500. AND if your lucky, maybe you can find a 2013 left over and save a chunk of change, as well!
I'm 225lbs currently, was up to 235 last summer. You can ride anything you want, for the most part
Just make sure the wheels are 32 spoke, 3x build. I have a mountain bike with a fork that is softer than it *should* be for my weight. It works just fine, though. I ride a road bike, no issues! Definitely get a quality bike. You will enjoy it more, and you will RIDE it more.
Things I would look for:
-Shimano Deore rear derailleur (mtb) or Sora rear derailleur (road bike/hybrid). Anything lower is usually of less quality. Cheap SRAM is just that, cheap. Their good stuff is fine, but I'll never own lower end SRAM.
-Rim Brakes (keeps cost down, and allows other components to be of higher grade)
-Proper size. I'm about the same size, and a 16-18" mtb frame is a good start, or a 52-54cm road bike. If your going off-road, the smaller frame size gives more...clearance
-IF used, has it been taken care of? If the chain is rusty or the wheels don't spin smooth, instantly walk away. Even when I mountain biked a lot, my bike was kept clean, and lubed.
-32 hole 3x rims! Most bikes around $500 have this, though!