For $300, you will find NOTHING new, as far as mountain bikes go, that will stand up to your weight.
Try eBay. Look for a 1-3 year old Gary Fisher, Trek, Specialized, Giant, Cannondale, or Haro hardtail bike with a decent front suspension fork.
If you can find a used bike from a reputable brand with a steel frame, that'd be even better. Good luck. Let us know what you end up with.