Thanks for your interest in Dialogue Plugin. 
You can add your own fields to the events/conditions and add blueprint logic to process them. This way, you can trigger any events/conditions really - playing animations or granting items. The documentation already covers that, and we plan to make a video tutorial as well.
Sound field’s type is USoundBase and Voiceover is UDialogueVoice, both native UE4 types. You can use either of them for voiceover, UDialogueVoice is a bit easier to use if you have different NPC recorded voice depending on player’s gender or language.
Not sure what you mean by that, but if you want to grant a player gold or experience, or do any other gameplay action, you can do it via events (see documentation link above).
All user interface in Dialogue Plugin is handled by a UMG widget. So if you’re familiar with UMG, you can freely customize its appearance and controls.
If you already have a text to voice code/plugin, you can use it in the UMG widget that displays the dialogue.
Yes to both.