J109s are very racy expensive boats that have a good following in the racing scene in the Solent with an active importer and class association. As to whether they are "strongly built" that depends on what you are going to do with it.
Very, very different kind of boat from the last ones you asked about!
I cant remember where I read it recently but someone was saying that J boat quality didnt match their price level. Certainly there is one boat in our club with a very nasty moon shaped crack round a chain plate and whilst it has been raced, it hasnt been (to my limited knowledge) thrashed.