Originally Posted by SnowJob

Well, you probably aren't going to be able to get something with full suspension for 300, so that shouldn't be a concern. If you find something with a front shock try it out and see if you like it. Since you are looking for a commuter bike I would stay away from front shocks, especially on used bikes. They're liable to be squishy and will definitely steal some of your effort as you pedal.

I don't know if mtbs are more or less likely to get stolen. Obviously flashy road bikes are the high value targets, but in my experience thieves look for the bikes that are easiest to get. So when you get a bike also get a u-lock and learn how to lock both your wheels to your frame and lock the frame to something durable.

Are you planning on leaving your bike outside overnight or do you have a secure place to store it inside?

BTW local bike shops are usually fonts of wisdom and have employees who will gladly answer your questions without any obligation to purchase something from them. They'll help you get a bike that fits right. Plus, if you purchase a bike from them you'll be able to bring it in periodically for check-ups or if you hear weird noises. I've found that to be a great benefit to buying from a local shop.