If you choose to be a coordinator in a Dynasty, you'll only control one side of the ball. You will still have to do everything else a head coach is required to do (like recruit, set the depth chart, etc) but when you play a game, you'll only control your side. So if you are an Offensive Coordinator, you'll play offense. You will be forced to simulate everything else (including special teams).
You can also choose to be a Head Coach from the start. If you do this, you'll play the game just like normal.
You can create a player and put him on a team before you start a Dynasty or you can create a prospect to put in the recruiting pool and try to bring him to your team.
It really depends on your team and the team your going up against. When I started a Dynasty as Hawai'i, I saw a lot of games in the 30s and 40s. It was going back and forth all over the place. When I played as Mississippi State (in the SEC, known for defense) I saw a lot of games in the 10s and 20s... so it varies. From my experience, though, there seems to be more high scoring games than not.
You can download rosters with the players names (just check out the NCAA Football Rosters sub-forum on here) and use them.