Umbra may be the most powerful weapon you can get, but you cannot buy it. Possibly the most powerful weapon you can buy is an Akaviri Warblade you can buy from Jensine which has damage of 16 and also...
First you have to have a filled Soul Gem,Then go to an Enchanting Station like the ones in the Arcane univercity. you can also use a variety of game mods on the net to do much the same for much less...
Some of the best weapons are Goldbrand (Boethia Daedric Quest Reward) and Umbra (Clavicius Vile Quest) Although you can make a truly powerful weapon yourself by enchanting it with a powerful Sigil...
Poisons can be used simply just my using them like you would a normal potion, only it will give you the option to add it to your weapon ( bow or melee ) it will work for only one use though.
Umbra! if you go to the Alyeid Ruin Vindasel, it is south-westish of the Imperial City, you can make your way through it and find "Umbra". If you manage to kill her, you get her extremely powerful...