its very common for the door stay to break (the thing that keeps the door open in certain positions), the previous owner possibly had that changed rather than the door pins. Either way you can often tell if new parts have been fitted. look for scratches or marks.
when lifting the door with a jack you should try getting it as high as you can before tightening the bolts, and jack from the handle side of the door rather than near the hinge. it will settle when you release the jack. is the door sitting low all along its length or just at the back/front?