i had exactly the same with my phone. replacing the usb port of the i9100 helped. i paid 20 € for a new original! one (i think you can get them a little bit cheaper, depends on your location). be careful not to buy fake usb ports from china.
i was afraid of disassembling too. but with the video guide it is simple and easy. the most "problem" i had, was to open the cover. do not force it, be gentle. it takes some time.
you will need a very small screwdriver. you do not need a original from ebay, just go to the next walmart or whatever you have and buy one