Hi,
There is a thread a couple of posts down you should look at 'Considering buying an ISF'. Lots of information on there.
Rust wise the IS holds up well and there are no horror stories. I've had all the plastics removed from under mine and the car treated for rust and sealed up. It was very tidy underneath with most of the rust assigned to the arms, etc, and not the chassis, but it was still very clean.
The water pump should not be a problem now and all cars should have been addressed by now.
Look for 'ticking' on cold start as the biggest costly issue that appears most places are the manifolds. They leak after a while due to cracking. On the plus side upgrading the headers is a good thing whilst you're there. Some people keep driving with them cracked as you can do with no problems, so it's not an immediate repair.
Other than that check the radiator over and prepare to replace. The gearbox coolant runs through the rad and when it corrods from the inside, it's terminal for the gearbox. This is a design throughout most Lexus models, and isn't a common issue, but worth noting.
Modifications wise, there are a lot of suspension upgrades and coilover options. Depending on what you want to modify, be prepared to shop overseas if you want to push the boat out. I'd imagine similar to options for the Monaro, the ISF doesn't have a massive amount of support UK wise.