Unless reconditioned by an expert shop and under warranty, buying a used TD5 engine is like walking through a minefield. Hard to know what you're getting really. The abuse, tweaking and neglect that these machines are generally subjected to are beyond comprehension, besides the fact that they are pretty old by now. If yours worked without problems for 7 years, I'd remove the top end and examine the cylinders and pistons from that angle. If it's a liner problem, you'll be able to see it more clearly. If it's just the main bearings, changing them is not a complicated job, even if done with the engine in situ and with the crankshaft in place (). You'll never know, you might get another 7 years out of that engine ! But if you choose to replace it, change that oil pump bolt, even if the seller says that it's already done. Thread lock it and let it cure for 24 hours before filling with oil.