1. You'll need to plan on spending at least $550 to make your plan work. Realistically, $600-$650. The UM15 is $225, plus $330 for the NX3000D. Add in the cost of speakons, wiring, connectors, etc. you'll be well over $550, but you can use the $50 promo. You'll still need to factor in the cost of a sheet of MDF, fasteners and adhesive. And this is assuming you don't finish the cabinet with paint or duratex.
2. As of right now, you cannot download the DSP tuning software from Behringer, so you'll have to manually tune it through the control panel on the amp.