Yes, building portals to dungeons will be easy. I am actually adding those portals to the starter project right now, so they will come included and you can modify them as needed for your type of game.
Server requirements are definitely a concern when working on the projected expenses in your business plan. However, I am not so sure that adding more connected players to a server will significantly increases the memory usage. Adding players should increase network usage and CPU usage, but my guess is that it will only have small effect on memory usage.
Right now I am working hard on getting an early release of my system out, so that I can get some developers to help me test it and provide feedback. After that I plan on spinning up some servers on AWS where I will test UE4 dedicated server performance and then I can give you a better idea of what your projected costs will be. However, as you say, in the end it all comes down to how well you build your game.
From the little bit of performance testing I have done, the only area I am concerned about is AI. Using the AI system in UE4 seems to hit the CPU really hard even with just a few dozen running at once. My guess is that if your game needs a lot of AI mobs you will have to write your game so it only runs AI when it is in range of someone viewing it. This is one of the reasons that for my game I opted to have smaller maps and then just chain a ton of them together.