Well, they arent essential I suppose, but they keep dirt and small stones away from the pivotballs- helps prevent excessive wear and the slop that causes. As for the A-arms, you can save a bit of money by just dremeling some clearance onto them, in the indicated areas:
The important things are the 3.3spec knuckles, turnbuckles (either from the new emaxx or the tmaxx3.3), and the CVDs. I'd also suggest some metal 14mm whell hexes, as the plastic ones flex and seem to be causing broken stub-axles. Drop the suspension low and add some stiffer springs, and you'll be ready to rock. Oh, and a good strong steering servo wont go amiss either, anything with over 250 oz/in of torque will do nicely.