Hey Nick, welcome to SAU.
In general Skylines kept stock are fairly reliable depending on how they have been treated, especially an NA one. There are a few common problems with them which aren't particularly difficult to diagnose and fix, failing coil packs is a big one. Other than major servicing (timing belt is the big one) the maintenance is easy and even timing belts are straightforward if you have some mechanical skills.
When buying one check the condition of the rails and I'd also strongly recommend doing a compression test.