a lot of the new golfs with highish mileage will have been used on the motorway so no real harm will be done unless really thrashed, i have a 2001 2.0 gti with 15,000 miles and it drinks oil. remember a 99 car could be 5 years old and with the average mileage per year going up (they say about 15,000 miles a year, it used to be 12,000) thats an average of 75,000 miles so if you can get one with 50-60,000 thousand on you`ll be safe. i would still go for a 1.8gti and get the AA to check it