As mentioned, these cars are extremely reliable if well-maintained. I've had mine for over 5 years now, and besides brakes and tyres, has needed nothing outside of scheduled servicing.
Also, during that time its apparently only lost a few hundred quid per year in depreciation, so it's hard to tell what a fair price is these days. But, given the state of the used market and knowing what these cars are like, I'd probably be comfortable paying that kind of price for a well-maintained example in really nice condition.
A few things to look out for;
If you can hear the car get started from cold, listen out for a rattle as the VVT actuators are known to make a rattling sound over time, especially if oil changes aren't observed, or poor quality oil has been used.
The problem isn't actually that serious and can even be reversed by flushing the engine and making more frequent oil changes.
Besides that, the engine should be really quiet, smooth and free-revving. A faint "ticking" sound will be audible but this is normal due to the nature of it's fuel delivery process (direct injection).
As mentioned, the spark plugs ought to have been changed at least once, and actually twice if going by Lexus guidelines (every 60K, or 6 years), so use that as a bargaining chip if there's no evidence (receipts etc) to suggest they've been done.
The plugs can be bought for around £60-70 and the change itself is not terrible to DIY, but involves disassembling the intake manifold with a few awkward to access bolts, so it can be quite costly if done by a garage.
The gearbox is also known to be a strong unit and once up to temperature, should be silky smooth with gearchanges being barely noticeable.
Apart from that, the IS is an extremely quiet experience inside the cabin, so any noises present should definitely be questioned.