Those are the correct MIP and RPM parts, yes. Interestingly, I never noticed RPM makes shock towers for the RC18 platform; I might have to pick up a set of those too.
I think the bumper will clear the body just fine, but I don't think it's really necessary to get the bumper in the first place. This vehicle has so little inertia that it's super-easy to stop, and it doesn't really plow into obstacles the way larger vehicles do.
I don't know of any narrower wheels that will fit, sorry.
If I had to spend $20 more, I'd get a set of front hubs and caster blocks from AsiaTees:
http://www.asiatees.com/display?Team...id=55875&pid=1
http://www.asiatees.com/display?Team...id=55876&pid=1
And then I'd suck it up and spend another $10 on aluminum steering bellcranks to improve steering response:
http://www.asiatees.com/display?Team...id=55515&pid=1