SQL Server add auto increment primary key to existing table

You can't "turn on" the IDENTITY: it's a table rebuild.

If you don't care about the number order, you'd add the column, NOT NULL, with IDENTITY in one go. 150k rows isn't a lot.

If you need to preserve some number order, then add the numbers accordingly. Then use the SSMS table designer to set the IDENTITY property. This allows you to generate a script which will do the column drop/add/keep numbers/reseed for you.

No - you have to do it the other way around: add it right from the get go as INT IDENTITY - it will be filled with identity values when you do this:

ALTER TABLE dbo.YourTable
   ADD ID INT IDENTITY

and then you can make it the primary key:

ALTER TABLE dbo.YourTable
   ADD CONSTRAINT PK_YourTable
   PRIMARY KEY(ID)

or if you prefer to do all in one step:

ALTER TABLE dbo.YourTable
   ADD ID INT IDENTITY
       CONSTRAINT PK_YourTable PRIMARY KEY CLUSTERED