You will definately be disappointed and angry with the price you will be quoted on selling your IS200, been there with MK Lexus, wanted the new 250IS but they were struggling to offer me 9K, mine is a 51 plate with 30K immaculate in every way. I bought mine one year ago in Hadleigh for 12,850 18k on the clock, so one year on! and a heafty loss... But I like it and will be keeping it for a while yet... No real choice I suppose>
Just managed to scrape 11.5k for my 53 IS Sport with 26k on the clock. I used Glass's on the web as a datum. It includes private sale price as well. Also you could look at Parkers on the web for free although the prices tend to be a bit on the high side.