People make to much of the lastest F18 models.
Just sail your Inter-18, if you are not regulary sailing in the top 20 % of a competitive fleet then there is absolutely no reason to sell your Inter-18.
There are still enough Inter-18's racing overhere. And all older F18 boats get a new lease on life when they are bought and owned by new sailors or sailing looking to get into the F18 class for an affordable price.
Sadly the US inter-18 have some modifications that puts them out of the F18 fleet, this maybe be hurting resale value, but not the fact that newer model F18 came out.
Wouter