I have decided that I am getting a bike with my second paycheck. ( 2 and a half weeks from today)

Looking on craigslist it seems that about 200 is the going price for a bike worth buying. (I want to avoid getting a new bike till I learn some more about all this stuff and can make a better purchase down the road.)

So to my biking friends out there, and I am looking at you, mine BigSis gnat23; What do I need to know, what should I look for and is a soft budge of 200 reasonable? Makes or models I should look for or avoid? etc; I think I would want something more of a mountain bike than a road bike... or do I ?

Things to keep in mind. I have a daily commute of only 3 miles each way, most of the trip has bike lanes, I am 6'1" at ~255-260, currently in probably the worst shape of my life. But I do have those "great" fat man calves and thighs ... so I don't think I'll die biking ... I hope

Austin in general is pretty bike friendly (at least compared to Dallas)

Other gear, I like the helmet that radixx suggested I assume I'll need the required lights for night use, any other safety gear I should look in to?

And i am started it with mexican food at 6am