The transfer from PS3 to PS4 is pretty easy, once you have the PS4 version of the game on your service account, ther is an option to transfer your save data to the PS4. I think SE has a web page that explains it better than I can.

You do have to buy both the base game (ARR) and Heavensward, but I believe that there are bundled versions where you are buying one package that includes both ARR and HW.

BTW, you will have an advantage (dubious though it may be) over those who used the free upgrade path. With the free upgrade to PS4 campaign, you gave up your PS3 license for the game, and transfer to PS4. So you can't go back and play on PS3. However, if you have purchased a PS4 license as well as the PS3 one, you can still play ARR on that old PS3...see, dubious advantage...