Welcome aboard.
£6K should see you in a 350Z easily enough.
Plenty of early models available in that price range although they will tend to be higher mileage, imports, non-GT models & some auto box examples.
Don't let the higher mileage examples worry you too much though providing it has a FSH and has been maintained according to the service schedule these engines are good to go well beyond 100K miles.
Service schedule here: http://www.350z-uk.com/topic/31110-service-schedule-for-a-350z/
Buyers guide here: http://www.350z-uk.com/topic/77426-buyers-guide/
Guide for new members/potential owners: http://www.350z-uk.com/topic/22880-guide-for-new-memberspotential-buyers/
Zeds for sale on here: http://www.350z-uk.com/forum/51-zeds-for-sale/