I'd recommend toasting at least a little bit, depending on what style of beer you're trying for. Otherwise you'll get a bland, starchy taste that won't be very pleasant. For a 2.5 gallon batch, at least 2 lbs. 3 lbs would probably not be too much, but soak them and then toast them (helps give a bready flavor, rather than a bland starchy one). Steep them for 30 min to an hour, around 150°F. Do not boil.
DO NOT add sweet potato flour to your boil! Get some sweet potatoes, put 'em through a blender or food processor (or hand-grate them), and add them to the oats when you steep. You can also try roasting them in the oven first, and then pureeing, if you just want to get their flavors and sugars and aren't hoping to try to use their enzymes to convert the oats.