Iphone 3GS used to die abruptly quite often. After researching for a while, figured that it could be due to the battery. Also had an issue with the phone's camera, that it was not being recognized.
So, bought new battery & camera from Ifixit, replaced them as per the guide available at their website, which was fairly easy.
My Iphone is back to life and working well so far
followed the guide available to replace the faulty battery and camera. it was a flawless execution with the precision instruction set available.
no specific advise as such. If you have a good stable hands, ready instructions carefully and follow them, the job should be done in 20min