I'll go out on a limb and predict that you'll be reeeaally happy with the Sport. It has a proven track-record and I doubt that you'll feel the need to make any upgrades or replace anything anytime soon. Again... servo will likely be the one exception and lord help me, when that servo starts glitching or dies, don't replace it with another Traxxas servo. Swapping out a servo is very easy too btw.
Not sure if this was mentioned or not but be sure to have a 2mm hex-driver on hand at the very least. Using hex-keys for basically anything will get old very quickly. Pretty sure the Sport comes with some basic tools and if so, the included nut-driver will come in handy for removing/ installing wheels.
Sounds like you're approaching this thing the right way though... not throwing a bunch of aftermarket parts at this rig before you've even driven it. Drive it and have fun with it. As you get more time with it, you'll begin to get a better feel for what you may at some point want to address... may want to play around with different springs, tire/ wheel/ foam combinations, etc... or you may feel like adding some low COG weight. But any/ all of that can be done down the road after you get more familliar with the performance and handling characteristics of this rig.