Give the 99 a serious test drive and see if all the talk about heavy rear ends on cornering have any merit. You should be able to tell pretty quickly if you snap through the gears if there is any accereration difference over your Boxster. Personally, I would not go for such a high milage used car, but that is just because I would like to keep it for quite some time. But I sure understand about finances. BTW, I am glad to see Boxsters out there with over 1000k.
Ed