what about the body? other parts to wear out over time? the brakes, fuel pump ect cuz the body is 16+ years old now. but yeah, if the interior is torn out, and you need 2k of body work ...

AND its not even running. hate to have bad news but this car is deff. not worth 4k.
i would say 5-6k is a good price for a good body/interior turbo. maybe more if things are pristine show room quality. so start from there and deduct for everything. maybe ask him to get the part to get it running, then make him an offer, cuz if its not just the plug, it could get costly very quickly.