Sorry but 20 hours is not a standard for a real RPG, especially when developer is trying to resurrect good old IE based game genre. I think that 100 hours (all quests and sub missions) is a decent value.
First of all full voice acting with shorten the game greatly, secondly the graphic design - please do not attempt to make ultra realistic high end gfx. This will only consume development time and will shorten the game greatly.
Please try to stick to the basics, I wouldn't mind to have text based npc dialog and 2D graphics.