I'm looking at an edition 30 which was registered in April 2007. It has 16.5k miles on the clock and has never been serviced. The dealer is telling me it's due a service now "the light was on when I picked the car up".
OK, that is feasable. If you have read and digested correctly the info on the VW LongLife regime, then you should know that the car can go upto 24 months (2 years, for those running out of fingers and toes!

) - OR 20k miles (for a petrol engine) - which ever occurs first. But you should also understand that the LongLife regime is actually 'variable' - and if the requirements of the LL regime are not met, then a service could be called for in as little as 12 months or 9600 miles!
From my readings on here a GTI reaching over 16k without the service light coming on is a bit optimistic to say the least.
Hmmmm, it
could be done. However, to reach 16.5k miles, you generally need to be doing around 20k miles per year (or more), and yours doesn't seem to fit that pattern.

Obviously there is no way of proving exactly when the service light came on but my first question is would you avoid this one??
Oh there is!

Contact the previous owner. Their details are on the V5C logbook.
And as for avoiding the car - well we can't really tell you one way or the other - without knowing some more detail from the previous owner.
If the answer to above is do not avoid then any clues as to how much this service would cost and also what is involved?
Prices can vary quite a bit. But firstly, you need to be aware that you not only need the 'Interval Service', but you will also need the 2yr 'Brake Fluid Change'. And don't forget that VW dealers will offer a price match - so get a couple of written quotes from other garages, and if they are cheaper, then the VW dealer will lower their prices too.

Oh and I would be looking at getting the servicing changed from longlife to time and distance.
Just ask the garage who services it to put 365 days into the service reminder settings, rather than the 730 days they would use for LongLife.
HTH