Woodburning Fireplace Chimney Renewal

I moved into a 50-year-old house a year ago that has a wood burning fireplace in the basement. The flue is through the middle of the house, and an "orphaned" water heater is sharing the flue. I know that that's a no-no, and I plan to get a water heater that vents out the side wall. I had a chimney guy come out and have a look, and the result was an estimate for about $2500 to install a stainless liner, insulation (vermiculite surrounding the liner)and replace a tile in the flue. The chimney is brick (as is the house) and in good condition. The previous owners used the fireplace and reported no issues (although they weren't aware of the water heater problem) and our home inspector indicated that the only problem was the water heater. Does $2500 seem like a lot of money for the job? Is it necessary to begin with, assuming that the flue is in otherwise good condition?
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