The "RS" version has slight better shocks, different Diff, Hex hubs up front and water "resistant" electronics in the RTR. It still needs upgrades...
It depends what you want to do with it and where you'll be running, Don't RPM if it's a racer. Super flexy, gets sloppy on the track although it is a physical impossibility to break, therefore great for bashing

I would defiantly say a RPM bumper ofr anything though!
For oil, depends what you run on. I always thicker oil up front to keep my steering response up. Thinner oil is better for bumpy, and thinker/lower ride height is better on less bumpy in general.
New tires are always a good way to start, new motor can be pricey, and like Guy said, Sway bars front and rear. They are a great handling upgrade. They keep your truck flat with more tire contact and less roll.