Ant has already covered the main points above, some other things that come to mind -
- Check the gearbox shifts smoothly and there is no resistance or crunching (no whining or synchro issues).
- Make sure the car drives straight and doesnt pull
- Check your clutch doesnt slip (Negotiation point?)
- Do the standard check for rust (as Ant said - common for R34's around brake light) but also around spoiler, doors, wheel arches, windows - etc
- Check the tyres have healthy tread (Negotiation point?)
- Check the bottom of the boot for any accident repairs, and make sure theres no water in there too. (If so - why the leak?)
- Check out the paint thoroughly. Does the texture match? Any paint runs or drips? Look for over spray on the rubbers and masking lines. Look for blending marks. How picky are you? Check it both, in the sun and shade. Will you be able to live with what you find?
- Do the standard history checks, make sure there is no money / loans associated with it and that there is no previous accident history