Summary:

-Find a LBS that cares for YOU, not the other way around.
-Ride different bikes from them until you are happy. Well temporarily happy anyway because you don't really know what kind of riding you are going to do.
-Have the shop fit the bike to you. Then go back as needed for further adjustments (as you get experience you will want the bike to be adjusted a bit differently). Some people I know say they have been back to the shop once a month for a year before everything meshed. Others didn't change their style much and it was good out the door. But, I bet you will change and the bike will need to change to match you.
-Buy New. Used bike only if the LBS is selling it and warrants it. The fact is you just don't know enough yet to buy a used bike on the street.
-Remember the bike is going to cost way more than you want to spend. But, you need to spend the money to get a good fitness and fun tool. Besides, if you change later you will be able to get a big chunk of your money back on resale.
-Don't let the myriad of bike brands overwhelm you. For our intents and purposes slight differences in the numbers of one maker to the other are meaningless. Something like distinctions without significant differences.
-The result will be something you can have fun on.