At one time there was indeed a US recall on the harmonic balancer for two part numbers across the years and chassis affected. There is a pretty good thread about it on BW: Harmonic Balancer Recall: Key Points - Benzworld.org - Mercedes-Benz Discussion Forum

I would have also thought that all of the vehicles affected should have been repaired by now, but if you live in the US and have one of the affected part numbers (just check, that's pretty easy) then call the stealer and see what the next step is regarding the recall. It never hurts to ask, after all.

glenmore, there is no maintenance interval, as such, for the HB, whether or not replaced previously, regardless of the reason. However, every HB will ultimately fail (just like every timing chain will ultimately fail), and should be inspected periodically for signs of deterioration. Wobbling usually is a bad sign, too.

The HB is not that expensive and is not that difficult to change, particularly on the M112 with the electric fan, so I wouldn't let that fact alone stand in the way of a purchase. Combined with a lack of service, records, though, it might reflect a vehicle that has been neglected (purportedly regular service would have also found and repaired the HB).

Good luck and happy shopping, WhiskeyDan.