I would say that in this day in age, buying a used K with well-documented maintenance records is the exception rather than the norm.
I was fortunate enough to be able to contact the PPO (previous, previous, owner), who owned it for much longer than the owner immediately previous to me. He had really no obligation to deal with me, but he was helpful in telling me at least what had been done recently, and what hadn't. Having said that, I would have still bought it in any event.
Red flags for me are usually RTV sealant and JB Weld, zip ties and other garbage holding things together. Sometimes a quick visual inspection can tell you most of what a written service record would.