1. $300 won't get you much. I would say a good $500 would be a good beginner bike. Front suspension only please, full suspension is a waste of money under $700
2. Anything from (Insert all purpose sport store here) like Dicks will sell bikes not up to the quality you can get for the same price at a bike shop.
3. I weighed 300 lbs on a lightweight road bike. Lost 100 lbs. Unless you're going to jump 40 feet off a cliff and hope to land safely, your weight is not a issue on any bike.
4. 29ers are advertised to people like you who are new. I know only one person out of forty or so that I ride with that have a 29er. They are heavy, clunky and too cumbersome to really offer a good ride.
5. Are you good with your hands? Can you assemble a bike? Follow instructions?
#5 will save you a lot of money on a beginner bike if you purchase off a bike store like BikesDirect. They sell bikes just like what Trek or Specialized might offer, but not the BIG NAME BRAND on the paint and for over 50% off. THis can save you money if you can follow simple instructions off youtube of how to assemble a new bike, service the derailliers, service brakes and everything else.
6. I would say about $200 for a Helmet GET ONE I DON'T CARE!!! BUY A HELMET!!! IT WILL SAVE YOUR LIFE!!!, a good multi tool, gloves, air pump, patch kit, and anything else you might need.
7. I said before, a good start up is $700, here is a good deal you can get that would be under the price but still high in value and worth every penny, as long as you assemble it yourself...
$300 and a great bike.
Learn your sizing, I fit 17" nice and 19" great and I'm 6ft
Now you need a Helmet, gloves and a pump. I have the helmet and the air pump, gloves are up to you.
Make sure to get the black and grey, I got the blue before and had to return it
You could probably share the pump and learn to put together a bike together, aw loving moment.
I guess I got you up to $400 then, how's that?