I would wait until you get on board to buy an Internet package. First use your Elite minutes and then you can buy a package if you are satisfied with the quality. It will be pro-rated for the cruise.
As for Specialty Restaurants, if you have particular days and times you want to dine, then book now. If you have a package, you will have to wait until you are on the ship and the times and days you want may not be available. You definitely can use OBC if issued by Celebrity. Go to Manage My Reservation and upon check-out, make sure you use you OBC as an option.