I would buy used on craig's list. That way if you don't like your bike or want to upgrade.. you don't lose much money. Many of the cheaper newer bikes are made out of aluminum... which can ride harsh. I would get a nice rigid steel frame from the 80's or early 90's. something like an early Stumpjumper. If you live in a relatively flat area, set it up with one chain ring and a chain guard protector. Fenders are also good when going through puddles even if it isn't raining, and may help you keep your clothes cleaner. Also good dynamo hub lighting is getting cheaper and cheaper.

I would also set some money in my budget for tools for basic maintenance. There are many good videos on Youtube about bicycle maintenance. No need to pay a LBS on things which you can easily do yourself.