You will need to have 10,000 coins at the end of the DLC and have a low chaos rating. Between the four missions in the DLC there is a total of 10,705 (500, 1655, 3520, 2030) coin, plus a starting 3000 in you start a new save and do not bring over a save from the previous The Knife of Dunwall DLC; which obviously doesn't provide you much room to mess up. You can attempt to go through the game on a new save so you have a starting 3000 coins, and just make sure you get enough in each mission...Or, you can play through The Knife of Dunwall DLC and get as much coin as you can (I recommend at least 5000) to bring over to Brigmore Witches, for a nice head start.
Some perks that may help you along the way are Void Gaze II, which will help you see loot through walls and from afar, and Pull, which will reel in coin, ingots, ammo, and consumables from a distance. Also, if you are planning on running through The Knife of Dunwall to make it a little easier on you, feel free to use the coin guide HERE to prevent unnecessary searching. Goodluck! Oh, and don't buy anything!