A lot.
I dailed my GT-R for a while, it's not fun spending $100 per week on petrol. Mine's has essentially a cat back and suspension, so reliability wise it's been very good (only had issues with the AFM).
IMO look for one that's been well maintained, and as close to standard as possible. Basically don't worry about km as they're most likely wound back, only use it as an indicator to judge the rest of the car.