With only 35,000km on the clock you should be OK for engine leaks etc. If it is leaking just don't buy it - simple.
I would be checking the service book, doesn't matter who has serviced it, as long as the oil has been changed reguarly. Also feel your way through the gears for any crunching, although 6 speeds are very tough, who knows it might have had a pretty hard life so far. Check the brake rotors for any cracks and life left (very expensive to replace). Check the welds around the engine bay and inside the boot to look for any accident damage.
Also
don't get ripped off with your tyres!! STI's come from factory with Bridgestone Potenza's DON'T accept any other tyre on it (unless it's better
) from a car yard, if it's a private sale than what you see is what you get.
Negotiate your price and then tell them all the points that you want fixed/replaced for the same price.