Right, of course that's fine with a newish low-mileage car but not an old banger.
My Landy was mostly built of second-hand parts. I got a superb 11,000 mile transfer box out of a drowned water board Defender for a fraction of the new price. It was fine once I had drained the stinky oil/water emulsion out of it and run it around the block three times filled with three changes of cheap engine oil to flush it out. Only the engine, chassis and bulkhead were original, the vehicle had done 225,000 miles with the RAF and had been rebuilt countless times with new and salvaged parts, we got the service record and it was 27 pages of detail on all the bits fitted and repairs carried out.