hmm if you mechanic is trusted and he suggests that the timing belt needs doing, then I would do it. If he says that it needs doing and it dosn't really and he just wants the money, then you will have to do it soonish for your 100,000k service anyway.. so you might as well. It may also be worth purchasing a timing belt kit, so that all the other things that might go wrong are replaced and you know they will be fine...
I recently imported an r32gtst, it had it's timing belt done at 104,000k's the car has done 150,000k's.. I'm going to do the whole kit anyway, even if it is unnesccary. Mainly for piece of mind. The thought of the belt snapping or a bearing seizing up makes me shudder
I suppose the main expense when looking at the timing belt is the labour, as it takes a while to do. Take a look at the r33 service manual, it will show you how to do the timing belts there, if you feel up to it, you may find that you can save yourself a fairbit of dough..
good luck