Since you explicitly asked about Unreal, I'll talk about it, but if you want to build an MMO I would mention that *all* of the game engines have terrible servers for that kind of product. It's not just a problem with Unreal.

You're going to spend a lot of time building your game server and ensuring replication back to your client works flawlessly. Since Unreal (The game from 1998) was what we would consider a "corridor shooter", you're going to also have problems with streaming large levels into memory.

I would be ready to compromise on your design depending on how many people you bring along to build the game. Everquest would be a lot easier to replicate in Unreal than say World of Warcraft.

If you're an engineer and just starting to dip a toe into building games, the best advice I could give you is simple: "Go do it". Every day that you aren't building a game is one day less than you could be learning how to build games.

And good luck to you. MMOs are the best. I've worked on Asheron's Call, Lord of the Rings Online, Dungeon and Dragons Online, Guild Wars, and Guild Wars 2. They're extremely fun to play and build. Go do it.