once you learn to drive an smg is can be just as smooth as a manual. I would be very surprised if you regret getting it. I really enjoy mine.
Only the other day I thought I might get a new car but I still love driving what I have and mine has done 130,000 klms now and still very tight car no rattles ect,
I think you will be hard pressed to find anyone who has owned a well maintained M3 SMG to say a bad word about them.
Just make sure all the maintenance has been done as it can be expensive to to play catch up on brakes, discs and having the valves shimmed.
Good luck with your purchase