All the 30" gravely decks (at least the older ones, I don't know about clamshell decks) could be used to mow either well-behaved grass, or rougher stuff. They are direct drive, and thus better for brush work than something flimsier. That doesn't mean all the 30" decks are as good at ithow do I tell it it’s a mower deck or brush hog?
The best deck, IMHO, is the MA106. That's the one with the big pipe plug on top of the gearbox. You may also run across the old-style "left hand" deck, which unlike the others has blades which turn counter-clockwise when viewed from the top. You also may find the MA58A and 185, which have smaller (1/2"?) plugs. Among other things, the 106 has a bulletproof gearbox, and a very sturdy 6-bolt flange to fasten the gearbox to the deck. The others used 4 bolts, and the decks were prone to cracking around the bolts.
Regardless of which deck, for brush work you'll want a heavy single blade, as opposed to the lighter "criss-cross" or "finish" blades. I've found that with a sharp blade, you get about the same cut on grass whether you have the brush blade or the finish blades.