Dolphin licenses will work for D8. No need to buy a new license. As for data migration - depending on module compatibility we will be providing migration tools. Some data might not be up for migration, just because D8 wouldn't have all the D7 features in the beginning. In short - if you want to build a site now - use D7 and don't test D8 later when it comes to see if it suits. We'll be supporting and developing D7 for a long time anyway. It's not going anywhere.