2.5 is known for doing ringlands and headgasket.
when the hg goes it can damge the crank bearings, maybe previous owner had the HG done, didnt do the bearings and now the bearings have gone. need ot get it to a specialist to get it checked out, but does sound terminal.
rebuild cost will depend on how bad the damage is inside and what needs to be replace, 4k is a round figure guess.