Auto update stats turned off for a user database on SQL Server
You have two questions:

1. Do you have to restart after setting auto update stats on? No.

2. Should you consider dropping the user created stats? If your statistics updates jobs (typically done with a maintenance plan or with Ola Hallengren's maintenance scripts) are taking longer than your maintenance window allows, then yes. SQL Server has to read the entire index (or table) to update each statistic, so you can end up with really long stats update jobs. If your maintenance window isn't a problem for you, then you can leave 'em as is.