there are more things to consider for using landscape in UEd: foliage, procedural spawning, landscape materials…many things you cant to with a static mesh as landscape geometry.
and also reconsider using real-world data for a game, as it is pretty boring and too large. either you have the assets to fill it up or come up with better content than the real nature is pretty boring in real scales.
just try that out in youre surrounding.