I'm honestly leaning more towards the Hutts here.
The Trade Feds probably keep records on everything, especially when a wealthy businessman is involved, too much of a paper trail for someone like Lennah I think. The Hutts on the other hand don't care who you are, so long as your credits are good.
Also I think Lennah would be outside of the Republic, it fits his 'loner' ways. When you need something done outside the Republic, you look to the Hutts.
(The Neimoidians strike me as the better choice for Lennah. He's a businessman, and so are they. They speak the same 'language'. The Hutts are really just criminals/dregs/thugs in a way, and don't seem like Lennah's type.
Plus the Niemoidians provide some illusion of legitamacy about the whole deal. If you're dealing with Hutts, you're up to no good. If you're dealing with the Niemoidians, you are not necessarily up to no good.)