To be perfectly honest, even cheap u-brakes will work fine. Yeah you can get expensive machined arms that are a bit stiffer and lighter, and fancier in colour than the cheaper brakes. And you can get some with low-profile hardware and fancy brake blocks but they don't make much difference. Find something you like the look of, at a sensible price, then take the time to set it up right. Iron out your frictional losses by lubing the cable/nipple seat, brake posts etc, use as little "spring" to pull the blocks away from the rim as possible, and get your blocks aligned well. Et voila.