1. If you stick with doing just the main-story quest, it theoretically shouldn't take to long. I've know people who have done it in less then a week and have heard of people doing it quicker then that. It could get a little boring though and getting a group in the Duty Finder might be hard. >w>()

2. As a tank, you shouldn't have a hard time getting a group together in the Duty Finder. I might recommend joining a Free Company and/or making friends that could play/help you though, just because the Duty Finder is a mixed bag, you might get veterans who are nice, you might get veterans who are jerks, you might get newer players who are willing to learn, you might get new players who aren't, etc.

3. If you are interested in XIV's story, bigger areas to explore, and want some more harder fights, then yes. If you are looking for a huge difference in game play, not so much... >w>()
For me, when it comes to MMOs, it grinds down to how social you are, how stimulated you are, and how much reward for work you feel you're getting.