The devs have said that they're planning a repository of sorts. Project threads sometimes have titles that include WIP or Released. If you only want finished products, stay away from the Works in Progress.
So far not everyone has marked their thread with WIP. I too would like to see a specific finished mod section.
As far as I know, the only two finished vehicles are Mythbuster's Caddy and dkutch's Campbell Moonbuggy. The Caddy: http://www.beamng.com/threads/437-1949-Cadillac-Series-62-Convertible-unofficial-build-thread The Buggy: http://www.beamng.com/threads/1217-BETA-WIP-2004-Campbell-Moonbuggy If there's any that I'm missing, let me know because I'm always looking to add more cars to my game