For an 18 year old car, condition is as important as mileage. A higher mileage car that has had regular/quality maintenance can be a better buy than a low mileage car that hasn't.
Classic example - my parents bought a new car about every 10 years and the car traded in usually had less than 20K miles................and in spite of the urging of me and my brother they did maintenance based on mileage not elapsed time so their cars got maybe three oil changes in 10 years.
'96 328iC "Marie"
R.I.P 14 November 2013