Yes! I forgot about adjustments. You'll usually have to have the cables adjusted within the first month. The cables start stretching with usage over time. I couldn't get into all of my gears so I needed to have the derailleur adjusted also.
Yes, it is possible but it will be close. For used bikes, a lot of bike shops will have some around. The bigger chains generally don't, though. Depends on where you are.
Will a used bike generally be as reliable as a new bike? If so would a place like Bikebarn or Sun and Ski Sports have them?