I was hoping that someone else with more experience with the F18 would chime in on this one as I am also new to F18s. I do have experience with Tornados that may transfer well.

I believe the Nacra is foam sandwich construction so you want to check for delamination in all the high stress areas. Beam attachment points, shroud attachment & bridle. Also look for trapeze hook bites on the sides that may have allowed water to enter and cause delamination.

I would ask how old the standing and running rigging are on the mast. If they are old replace them as a new mast will be equal to the cost of a used boat.

Check the foils for damage, and check the rudder castings for wear.

Check the sails and find out the age/condition.

There are several examples of F18s for sale here so you can make sure you are not over paying http://www.thebeachcats.com/classifieds/g1-catamarans.html

Here is a thread that talks about what I believe to be the model of Nacra F18 you are looking at http://www.catsailor.com/forums/ubbthreads.php?ubb=showflat&Number=234595#Post234595

This may not be all you should be looking for but is what I can offer.

Good luck

Happy sailing