Welcome to NICO!
The G is a great car with an engine that has a reputation for excellent reliability. At 78,000 miles that car has likely seen an easy life and is just getting started. Its highly unlikely that you'll see any major repairs or maintenance aside from a clutch (which may or may not happen, depends on the PO's driving habits and yours). Oil consumption is easy enough to keep in check, and by '07 the issue was corrected min-production. However, before you buy, you may want to take the VIN to a Nissan or Infiniti dealer and see if they have any records for the car- maintenance, oil changes, recalls, engine replacement, etc.
If I were in your shoes, I'd go out of my way to inform myself about the cars history and if it can't be confirmed, leave it alone and walk away if you're still worried. You can always get an '08 for a few grand more and some piece of mind.
Additionally, here is the TSB for the G35 oil consumption issue and process.
http://www.g35driver.com/forums/tsb-tec ... n-tsb.html