You will really enjoy the Steel Heart! It is impressive to say the least. The Leatherman is a great choice for a mutitool also. As for folders, if you already have an LCC, a Military, and a TNT - you have some damned good choices already!! I would probably just go with the LCC.
You mentioned needing to use the folder outdoors and getting grit in it so you might want to consider getting something relatively cheap with a strong lock. That way if you scratch it all up and get it all dirty, you won't worry about it. If you plan on getting it really really dirty, you might want to get something you can take apart and clean later.
I would have suggested the Spyderco Wegner, but you can't take that one apart and if you get it dirty, all you can do is hose it off and try to dig the stuff out of the handle.