I have the BOSE setup in my car, factory fitted and I thought I'd just share a few thoughts.
I think the BOSE setup is a good compared to most other car 'factory' setups I've heard - at least at the time I got my car. BUT compared to a custom set-up done properly, the BOSE wouldn't compare and would cost more
However, you say you are just buying the SUB
... if so, you will at least need the amp as JohnK mentioned.
I suggest you do a search here on BOSE to see problems with settings on the head unit to set the sound properly. Also I think in the BOSE set up, the tweeter 'BOSE' embelm doobreys on the doors don't house anything (haven't looked in mine) but maybe the normal speaker setups do? Speaker setup, therfore, may require more thought
Again I think this has been discussed here before so have a looky.
There are some experts in audio here, so hopefully they can confirm/comment and provide more in depth feedback.