Essential first mod to any used car you've just bought imo should always be a good service.
Eg: New oil/filter, air filter, clutch/brake/coolant fluid and possibly a change of spark plugs depending on whether they've been done or not.
That way you'll know for sure that all the basic things to keep the car in good running order have been covered and then you can start thinking about any upgrades afterwards.
350Z service schedule here: http://www.350z-uk.com/topic/31110-service-schedule-for-a-350z/