don't know much about the octane other than it runs on gas vs nitro for the 5.9
Keep in mind with a Savage the parts by HPI are no longer being produced and we are waiting on potential new ownership to get with the program and offer support again.
Here's a link to HPI for part numbers and manuals you can download for reference:
https://www.hpiracing.com/en/parts/112601/std
For the 5.9 I would say get some spare parts like diffs (front and rear are the same), dogbones, slipper pads, brake disks, shock bodies, shock caps, o-rings for the shocks, and clutch shoes depending on how long you plan on owning this. You may never end up breaking these parts if you don't abuse the truck, take it on jumps or crash it, but slipper pads, brake disks, and clutch shoes will wear over time.
One thing I would definitely look into upgrading is a hardened clutch bell (HPI or Robinson Racing), steel spur (Robinson Racing), with good aluminum clutch shoes like Dynamite Max Life shoes 5180 (avoid Integy for shoes, is made of a softer aluminum)
I'm running Robinson Racing Gen3 47T spur 7347 and 17T clutch bell. I've yet to wear out a hardened clutch bell or steel spur, just make sure you set the gear mesh properly and use loctite on the engine mounts. You can find plenty of videos online on "how to set r/c gear mesh" or "how to set spur gear mesh". There's very minimal play between the teeth, just enough to be able to hear the slightest tick when rotating back and forth the spur gear by hand. If for your application you feel the truck accelerates too slowly after a proper tune you can lower the tooth count on the clutch bell. If you're having a hard time keep the front end planted and it just wants to wheelie you can always go with a larger clutch bell for more top end speed and less acceleration.
And if you're unfamiliar with nitro engine tuning it's not difficult to tune and maintain, but you'll just need a glow plug igniter, spare glow plugs, and fuel. Look up on youtube "how to tune nitro engines". And if you have a cheap IR temperature gun to verify you're not running at a borderline too lean setting that would help.
