Theres no exact amount of km when the belt,balancer,water pump etc needs to be done. The more you maintain the coolant and oils etc,the longer components will last. If its hit 90,000km and you going to get a tune etc,it would be wise to change all your fluids,replace your fuel filter and even replace your turbo oil supply line filter,along with a thorough checkover of the whole vehicle. If you havent replaced your drivebelt before it wouldnt hurt to replace that also. If the water pump isnt noisy or leaking and all the pulleys are quiet,leave them. Chuck a set of new plugs in at smaller gap aswell and you should be right with your new mods and a tune.