IMO you only need spur gears and a backup one-way bearing for the rotostart (if it's pull, a backup pullstart would be wise..) - at least that was my experience with the Savage 25.

With the bigger engines a lot of people talk about breaking dogbones - but I still haven't don that with my X SS.. Nevertheless, I have some backups just in case, and based on what one hears anyway, it should be high on your list of parts to have on hand.. What I HAVE done, is break one upper adjustable arm, and rounded out the plastic heads on the rear turnbuckles.. To remedy the former, I went with the non-adjustable up front(included in X SS kit..), and for the latter I've ordered some Integy tierod eliminators..

Beyond that I don't really think you need anything else.. Yes, you COULD break this or that, but with the Savage, the basics are spur gears and dogbones...