Brocephus

In your price range, I concur with the above recommendations to use Craigslist, as well as to look into older 26" bikes with rigid forks. In my earlier riding days, I used to buy (new) $350 hardtail mtn bikes, (Treks,Specialized, GT, etc.) with low end Shimano Alivio/Acera stuff, and they all held up and ran surprisingly well, even in comparison to the much higher-end bikes I later had.
Also, don;t get too focused on chasing down a specific brand or model, these are just general suggestions, not hard-fast recommendations. Besides, it's gonna be nearly impossible to find a specific older used bike, in decent shape, and your size, so don;t even bother.
In a given price range, name-brand bikes tend to be much the same, your main concern should be condition, and correct size. You want a bike that was bought new, barely ridden, and stored in the garage for a few years, then posted on Craigslist, cheap.
You don;t necessarily need to be an expert to tell if a bike has been used a lot, or mistreated. Heavy road grime, rust, dings, bent/broken spokes, etc are all obvious indicators that anyone can spot.
I see decent old hardtail, rigid fork mtn bikes from Trek, Specialized, Fisher, etc on Atlanta's Craigslist, in the $120-175 range, all the time, so i assume most any large metro area will be similar. (BTW, where are you located? Some of us would be happy to take a look at your local Craigslist, and provide some recommendations, or warnings).
Also, concerning sizing, you don;t need to go to NASA to have the calculations done. For a first bike, for light,casual exercise, there's some lee-way here. You obviously don;t want a bike there you have the seat adjusted all the way down to the top tube, or a bike so small, you have the seat post pulled out beyond it's maximum safe level, but having a mtn bike frame that is a little larger or smaller than the math says is ideal, generally isn't a serious detriment, for a casual recreational rider ( back in the day, my low end mtn bikes were set up with road-friendly tires for street commuting, and I had 17",18" and even 19" frames, and was perfectly fine on them all.)
Another point: obviously it's a good idea to check a bike out, riding it around, and all that, but being a novice rider, that's not really going to be much use to you. A bike could be absolutely perfect and a great deal, but the previous owner have the seat adjusted too high or low, or the rear derailleur just need a small adjustment, or the tires need air and the chain need lube, or the brakes rubbing a little, and you'll think the bike is terrible, when it may not be. So a "test ride" is largely useless to an uninformed rider, unless everything happens to be perfect.
Also, check around and see if you have a freind or work acquaintance that's a bike guy, they'll generally be happy to help.