There are a few missions in SP where you get the chance to destroy battleships, but if you're talking about random battleship killing missions forget it. You dont even get a chance to kill a battleship after SP never mind getting a mission to do it.

There is an official server patch out soon but it doesnt do much. As for any sort of expansion patch... who knows if everyone crosses their fingers and prays maybe it'll happen

I dont know of any mod to just make battleships etc buyable. If you're up for it you could probably mod your own game by checking out the various tutorials on the editing forum. However, and i'm only going from what i've read cause i've not played it yet, FL Rebalance mod lets you buy gunboats and while it does change a bunch of other stuff it's stuff that needed changing. Also its designed specifically for SP.