I would like to use UDK for your project, but support of outdoors is very bad. In my experience (in March or earlier release), the editor when working with larger terrain level (3x2 kilometer) and only maps saving takes maybe a minute and light generation and the worst quality of the hour.
Between the sectors is impossible change code parameters (afaik) and letting the enemy move easily between sectors, plus I think the sectors still must, be defined in one big map. Moreover, thanks to this is not possible good outlook (view distance) and must use a stupid fog. Simple tool Unity havent such sort of problem, big island works properly.
I think this lack also prevents attempts to MMO in Unreal, create game as Oblivion or Fallout are in Unreal impossible.. Small Mass Effect levels look as bad joke.
Therefore, I ask if Epic plan a significant improvement in this area, because otherwise I'll have to look for another engine, which have majority of features worse, but big map is possible.
Edit: Streaming volumes are not the solution, it is an emergency solution, in addition according to the documentation last upgraded in 2007. Unreal is in general nice technology all, but some things are as from stones age. It looks that Epic is affraid touch this old code.
Actors transfer between levels is done very poorly.
Streaming also have to working inside editor for basic work in 1 big map.