If you're not used to cycling, it's hard to tell by just trying what a good size for you would be. I mean, I know when I'm on it, but I ride every day - you're going to have to cheat. Tip: go to a bike shop, have them size you and write down the numbers. Try a couple of bikes until you find something you like. Tell them you will think about it, and then turn around and see if you can find something similar on Craigslist. Yes, this isn't the 'nice' thing to do, but if you pick your neighbourhood bike shop they will later on make money off you on repairs, so boohoo for them.
About breaks and gears: never buy a bike without test driving it. You will soon find out if the breaks and gears work smoothly by using them. If you are not confident about this, bring somebody along who can test drive and judge. Also, look for obvious damage like rust (is especially bad on the chain and the gear blades), fraying cables, missing screws, dents etc. What's a good deal depends on what you're buying. 30 bucks is a bad deal for a bike that costs 60 at Walmart and is generally crappy so you'll hate riding it. 150 bucks is a good deal on a high-performance, high-quality bike that would cost 10 times as much in the store. If I were you, I would go with a relatively cheap model which has the features you need to get around in your environment, but nothing more than that. Lots of moving parts means lots of potential things that might break, and you want something you don't mind falling over on while you're learning. Only get something fancy when you've made cycling a habit, so at least you will really be getting your money's worth.