That black thing's the mech hanger, onto which you mount the tensioner.
If you don't mind the weight and want to save money, get hold of a cheap, shite derallieur and lock it out using the limiting screws, and by using a spoke where the cable would usually go. OR, make a spoke tensioner
Your hub is decent for an ordinary bike, but it's not up to trials standards. Rather than worrying about swapping hubs over, see if you can find a second hand pro II wheel for sale on here, then you can keep that wheel as a spare. It also saves you building it... and you're more likely to find a full wheel than a hub on its own.
Brake-wise, get some good pads, a good lever and a decent cable. A regrind would be for the best, if the pads are up to it. A decent lever would be one such as an Avid - http://www.tartybikes.co.uk/product.php?product_id=10091&category_id=71 or a Shimano - http://www.chainreactioncycles.com/Models.aspx?ModelID=39002