If the UMP is really what you want then get it, but just letting you know that it has a lot of knocks against it, despite it being a really awesome and sexy looky good. A gun isn't much good without mags. And mags aren't much good without something to carry them in. Carrying UMP mags on a chest rig is not very practical due to their length. You need them set up on drop leg rigs, which means more gear to buy, and few magazines can be carried.
Like the P90, there's a reason that there's not a lot of them in use. It's not because they're not awesome guns. It's because everything about them is so specialized that it's hard to get what you need to use with the gun. And even them, the long mags are cumbersome. I own a P90. I love everything about that gun. But I wouldn't game it as my main gun for those very reasons I'd recommend against a UMP as your main gaming gun. AK, AR or MP5 would be my recommendation.