Good morning and welcome to the club. If you get a pre March 2001 model, the road tax is less! The government changed the pricing around this time and the older models are only £225 for the year (don't quote me on the price, I'll get the link in a moment. Lol)
Make sure the cambelt has been changed if it has mileage over 60,000. Lexus initially recommended changing it ever 100,000 miles, but some issues were found in a small number of cars, so they updated their recommendation.
I have seen IS' on Autotrader for as little as £500 and some of the newer models' owners are asking over £3000 for.
So if you are looking at a cheaper tax 2001 model, aim for £1000 - £1500. A full service history and a clean car will be worth more.
Don't fret too much at the high mileage models. I bought mine with 125,000 miles on it. I didn't research, but have been lucky. Engine wise, if serviced regularly and the cambelt has been done, they should be fine for 200,000 miles +.