hello lexxus, when i first bought my sc400 w/107,000 miles, there was never a single tune up done on that, except oil changes....and looking at the oil and fluids, somehow i doubt they did anythign at all in the past year....but anyhow, my belt held up pretty good and it was close to 110k. definately dont wait until then.
check your belt. is it shiny? any cracks on either side? those are signs of a wearing down belt. i never take my cars to the shop, but i heard the sc400 will cost you approx 2 grand...maybe a little less for the sc300 since it's an inline and the water pump is so much more accessible than that of the v-8.