It does depend on what model it is i.e. navigator, styling kit but there is only a couple of hundred pounds difference, if you look at parkers guide put in the details it can give you an indication.
Front discs and pads on Ebay click £60 inc P&P.
Brembo front Discs click me £120 inc P&P
The crux of the matter is that the IS300 is quite rare and so to get hold of one is good going I would suggest taking off the cost of the work to be done and then a bit more and see what they say, I always like to go in a little bit rude with a low price as some people surprise you with what they'll take. I did offering £1000 under the asking price and got £700 off, on a private sale :D
The Lexus straight sixes are pretty bullet proof so having service history is good. I would check the cam-belt hasn't been done as they revised the cam-belt service from 100k it to 60k at some stage not sure when though. I checked mine the other day and it looked pretty new and mine is the same as the one you're looking at including miles, so must have been done.
It is a very good car and the power is not to be sniffed at, IMO it is a very fun and enjoyable car and worth getting your hands on.
Russ