I don't know how many is too many but have now owned 2 6's with rather indeterminate mileage. Carfax is totally without merit as for as mileage is concerned in MY opinion. If the engine was truly rebuilt 70k ago and I had documentation or knowledge as to EXACTLY what was done I would go completely on that and the physical appearance of the car. If the engine and Transmission are in good condition and there is no extensive rust damage you can maintain the rest within reason as far as I'm concerned. It does help to be mechanically inclined and have tools as with any repairs shop costs are high. Parts are reasonably priced from good sources and most are available promptly. I had a Ford 150 that had a damaged mudflap and it took 2 months to get one. The only parts problem with the Six is with body parts or appearance items in my experience.