I think the theory is for the 60k you do just the belt and tensioner, at 12k you do the belt tensioner, tensioner stud, waterpump, thermostat, cam and crank seals, and at 180 you replace the belt, tensioner, tensioner stud, idlers, and idler studs. don't cheap out on the tensioners, idlers and studs, I've seen the studs snap and the idlers lock up and take out the whole engine, and to find a 60k 90 model is very rare! if the price is decent snatch it up quick, I'd be tempted to do all the goodies on this 17 year old car even if the milage isn't up, but yeah courtesy sells kits, I believe z1 also sells a kit, but I have found nissan of gastonia
http://buynissanparts.com to be the cheapest and a NICO sponsor