Forget abotu looking at the KM's as stated above they are almost all wound back. For a real idea of KM look at things like the drivers seat, you will see wear on the outside hip support, also check the 'leather' on the handbrake boot, the wear on the gearstick and also on the steering wheel. This will tell you how much use the car has had
As for fuel consumption, if you really have to ask then it will cost too much. I have a big thread on fuel economy for R33 gts-t's, which will give you detailed info if you can be bothered finding it. Basically its all down to how you drive and if its city or freeway. When my cra was making 200rwkw i was struggling to get over 270km per tank, changed filters, plugs O2 sensor blah blah blah and it went up to about 320. My cra now has 250rwkw and i did a whole tank driving for economy (car only went above 12% inhector duty cycle 2-3 times) and hey presto i got 420km!!!
So as i said its all down to how you drive and the tune, thats if the filters and sensors are operating correctly.