1: The benefit of signing older players at L2 is because it keeps the costs of transfer fees down for when you play in higher levels later on in the game you have more $ to spend and also you can concerntrate on costs of building up your stadium/facilities, plus most teams you play in the lowest levels are abandoned or noobs that don't have any idea how to play the game so you don't need an amazing squad.
2: Auto-assign just applies the training skill points automatically whilst keeping an even balance of points distribution.