Another potential S30 owner. Good deal!
First, 260's were only made in 74 in the US. 1975 was the first year of the 280Z.
The biggest thing you want to look for is RUST! I don't know where you're located but unless it's sunny CA, AZ, or some other hot arid climate, you need to make sure the car is worth spending money on.
Almost everything else is secondary, except making sure it hasn't been wrecked. Look for body panels that don't lineup, poor repairs, fiberglass (bring a magnet.)
Look for rust in the following places:
Most importantly - the frame rails. If these are toast then the rest is usually not worth trying to save. Surface rust is no bid deal - rust from the inside out is what you want to look for.
Underneath the battery tray.
Underneath the hatch, especially in the corners of the lip where the hatch rests.
On the hatch itself, underneath the edge.
In the spare tire well.
The quarter panels.
Underneath the seats and the carpet. Don't be afraid to pull it up! If you are, you might regret it.
Some rust in these areas is common - just be aware that you'll need to take care of it ASAP and factor it into what the car is worth to you.
In my opinion, if the body is straight, clean, mostly clear of rust, you will be in good shape even if the engine is toast, the suspension is toast, etc. Everything else is easy to work on, easy to replace, but if you buy a car with rust in critical areas, that is a nightmare.
If it is running well and feels ok, expect to go over the brakes, suspension, fuel injection and spark systems before you can really drive it with confidence. Make sure to get his maintenance records - common things to replace/upgrade:
Brake lines, rear wheel cylinders, ujoints, ball joints, tie rod ends, struts, suspension bushings, clutch slave cylinder. See when the last time he replaced his plugs, wires, cap and rotor was, as well as his clutch, tires, diff fluid, transmission fluid.
Datsun made extremely reliable and fun cars, once you own it and work out whatever issues it has, I think you'll be happy!