I just bought a house with a sewer line problem

I just purchased a house in April 2008, the first week we had the house the main sewer/drain line froze (I live in Alaska). Which causes or bath to get backup. I got it thawed out. Tell the next week same thing. During this time we also has our washer and dryer moved down to the basement. So now low point is there. So when we freeze or get backed up water come out of the washer line. and floods my basement. After about 4 times, And $115.00/h to thaw it each time. I decided to get a plumber with a camera to see down the drain. To see if there might be a bigger problem.
And of course there is. There is a tree root that had grew into the sewer line. I need to get it fixed before winter. They quoted us $8000.00 to redo the whole line.
The previous owner did not disclose any of the sewer line problems. She did know about it. Because the many times we had the City come and check if it was frozen from there end. They would be out there almost weekly. I'm not sure if i have a leg to stand on. I HAVE NO IDEA WHAT TO DO! Someone please help.