I thought RPM made caster blocks for the N. Rusty... If they do, I'd use them. Pretty much any plastic part that RPM makes for the truck, I use. At least for suspension and chassis. Anything else I leave factory. My N. Rusty has RPM arms, bumpers, and shock towers. (I converted mine to use Jato arms and axles). The plastic stuff (even OEM) is pretty tough. If you break certain parts more often, try cooking your new plastic parts before installing them. It makes them a little more forgiving. Boil some water gently and pop your plastic parts into the water for 10-15 seconds or so. Let them cool completely, install, and you're set. I do this routinely with RPM stuff, but it works good on OEM stuff too.

If you get crazy and decide to put a 3.3 or other .18 sized engine into your N. Rusty, the Jato suspension conversion is a good stability upgrade for the crazy power of the bigger engines. It makes the rustler about 1/2-3/4" wider.