Lets be honest mate, not much is a "problem" with skylines apart from age.
Check the exhaust system for cracks.
Check the cooling system does not have oil in it
the oil does not smell like fuel when you look at its colour smell it.
seal for the light on the boot.
Drive it listen for any noises note the position, if they are anywhere near any of the wheels dont buy it (wheel hubs - when you speed up the sound will increase with speed).
Car shudders under hard braking, will require a quick fix which is cheap.
Ive had my R34 for 3 years now and all that has gone wrong is a cooling channel broke down and split. However when I went to buy a car from a dealer on parra road I had it checked first and the car came back faulty in so many ways it was not funny so I must tell you to get an inspection once you found the ride you like.
Oh watch out for those Nismo wings, they are f**ken ugly and people laugh at you with them!