It is probably a vacuum shift control problem. If it is, then I wouldn't worry about it. It probably can do it forever. As to whether to buy the car, the early Turbo-diesels are notorious for poor shifting. We usually make adjustments at least every 30k on these systems.
If you can find someone knowledgable on that trans, determining if this is the cause is simple. Fixing it and not causing new problems is not necessarily simple or achievable.
Bosch Master, ASE Master, L1
33 years MB technician