When I looked a having a liner put in my chimney 7 yrs ago (fortunately, it was not required) it came to between £1k and £2k to put in a double skinned stainless steel chimney liner. It is likely to be a lot more now, and even more if access and height are difficult.

Seems to me that if the repairs are not already reflected in the selling price, you should drop the price you offer. If this is not agreeable to the seller .... walk away! Richard