Hi -- In my view the running in period of a car is the most critical period of it's life, the engine oil and filter especially should be changed on time using the correct spec lubricants, to aid the longevity and condition of the engine.
So the service history should ideally be there to show this has been carried out correctly and on time, if there is no other way to prove this, then you should be offering less if you're willing to take the gamble, or considering walking away from the deal altogether.