Open bar is definitely included in the site fee of $600 for Baja Norte, so the only additional cost you have there is the plated or buffet per person fee. If you think you want to use Baja Norte as your site, you should email the wedding planner and let them know ahead of time. There can be multiple weddings in one day and so you want to be the first to reserve your site if you know what you want.
If you are having a private reception, you can reserve a table at a restaurant as your included semi-private dinner in your wedding package. If you are using the semi-private dinner for your reception, you can easily give one of the waiters at the buffet a tip to set you up a table in the back for all of your guests for a rehearsal dinner so you can all sit together. We did this multiple times on our trip and they were very accommodating.
Your choice for ceremony is the beach or gazebo. Depends on what you want- the beach is an amazing backdrop for photos, etc. The gazebo is a little more private.
As for reception sites, our wedding reception was at Baja Norte and I would recommend this- beautiful setting and private. But if you're having a semi-private reception I think the steakhouse would be good. We used the resort-selected DJ and he did a great job so I would just go with him. He looked through my iPod (I had some playlists for the reception and dancing) and used some of my music but mostly his and it was completely in line with things we wanted to listen to. Our guests danced straight through until 11pm when the reception ended.
Finally for photographers, we used Ana and Jerome, who were fabulous, although a little more expensive than the resort-designated photographer. They ended up paying the vendor fee to get in so we didn't have to. As I mentioned in a different post, we don't have photos back yet, but I was so happy with how attentive and organized they were, not missing out on any opportunities and really maximizing daylight hours for the best photos.