Yay, a fellow JoJo watcher! We need more JoJo threads.
It's like this, there are 2 OAV series.

The first one has 6 episodes. It portrays the end of series 3 (manga volumes roughly 25-28) featuring some other stuff which the fans wanted to see animated i.e the fight with D'arby the Gambler & GEB. It has Iggy and ends with Dio being destroyed.

The second one is newer and has 7 episodes. It portrays the beginning of series 3 (manga volumes roughly 12-18) and shows how the team met. In the end they defeat the stand Justice which is owned by Enya. This series has Hol Horse in it, but no Iggy.

The second one has no translation for the 7th episode, oddly enough. The DVDs you are talking about are all 13 episodes, the second series being released first since it's the prequel.

Hope this clears stuff up.

The torrent on anime4ever btw is hard to know which it is, since both series are OAVs.