Miles can be useful when judging a vehicle, but there are too many other important factors. A vehicle beat to death for 50,000 miles is not that different than a vehicle driven correctly for 200,000. As they say, a candle that burns twice as bright burns half as long. That can be especially true of transmissions. Maintenance is also critical, as well as the quality of the materials or parts used. Age can be important, but where and how is was stored can over-rule that. There is no "blanket" answer to your question. Another thing to consider is how you will drive it. I sold a 73 Grand Marquis to a kid years ago. I told him that the 1-2 shift has always felt sloppy for all the years I owned the car and cautioned him about hot-rodding. He blew the tranny within 200 miles, but had no ill will towards me because I gave him an honest warning.