Congratulations on the car.  I have had to put about a half cup of distilled water in my rear coolant tank in the trunk twice this summer.  Once was right after a 700mile non-stop drive.  The second time was last week after about 2months of 90*+ temperatures.  It seems to do the trick.  My understanding is that you should never have to change the coolant completely out.  But im sure there have been exceptions.   Take it from me a fellow 99 owner, save your pennies now.  Proper maint. is a must on these cars.  If you let things get by you'll regret it.  Minor issues tend to cost a considerable amount more for these cars.   Hope you have some sunshine tommorow to get her out. Take pics and post.