I run mine mostly on baseball diamonds and I'm using a Traxxas VXL 3S - 3500kv* setup with center diff.
It's a nice change from the short-course trucks, handles similar but looks better (IMO). :joyful:
*The Horror!! - I know :deadhorse
If I didn't already have one, I'd just get the RTR, it has most of the upgrades I'd want, and (if you just have to have something different) the electronics could be sold pretty easily on ebay... or simply used until they fail.
I haven't seen any of the Rally parts (Arms, links, drive shafts), on Jenny's RC, but that may change now.
Diffs, bulk heads, center shaft are same as Slash. I replaced the clutch setup with a center differential on mine.
I'm not so sure your $200 'upgrade' radio has proven it's value either... you pays your money and you takes your chances.
Arms, drive shafts and links are shorter on the Rally vs the Slash.