yea our r31 is around the 200,000km mark.
Belts -
The timing belt should be changed every 100,000kms but it will most likely last past 100,000 for a while depending on when you replaced the older one, but its best to replace it.
Im getting all the belts on mine changed, the power steering one is starting to fray and i think the timing belt isnt much better.
plugs - I would just buy the copper ones, they do just as good and they cost alot less but i think you get more kms out of the iridiums.
It would be a good idea to change the petrol filter. Its probably blocked up with shit, it will make a fair difference after you change from a bad filter to a new one.
Other things while your at it -
Perhaps change the air filter and clean the throttle body.
An oil + filter change if it needs one - some good quality mineral oil would be fine and its not to expensive.
Thats most of the stuff thats been done on my r31, but i still need to change all the radiator and heater hoses as they are getting crunchy and the radiator needs flushing. Plus i think the thermostat has stuck closed as the temp needle gets to high very often.