Hello and welcome to the Forum. I'd say a piston, a bolt, a recoil spring, perhaps a cam pin, a firing pin, and depending if you change out the trigger with something else, a trigger spring, and perhaps a butt stock - depending on how rough you think you might be on it. I had doubts about it's rigidity, and still do somewhat - but the fact is, no one's is breaking. Take this sound advice from someone who learned the hard way. I'm a believer in having spare parts, but the SCAR really doesn't break. I bought a TON of spare parts when I first got mine, and spent about two months selling them like 5 years later. I was fortunate enough to get probably 90 % of my money back - but man oh man, I did a LOT of shipping. I had bought EVERYTHING in sets of 3. The number of parts I've replaced on my 17...ZERO ! Perhaps it's time for a recoil spring, but it's a very durable rifle.
I'd say receiver screws and a set of the heavier barrel extension screws used to tighten down the barrel. Get a torque wrench for removing and reinstalling the barrel - 62 inch pounds. Quite honestly, I like my cheap Wheeler Fat Wrench better than the expensive alternatives. The receiver screws are 32 inch pounds I believe - it's been a while since I've removed any. Those are intended to be one time use only - replaced each time you remove them. Use heat before removing them - they strip out easily. And use green loc-tite 603 on replacing them. Almost impossible to find in stores - so do yourself a favor and just order it online. That's the loc tite FN uses. The receiver screws are a rip off at like a dollar each. Most hardware stores don't carry screws that small. I found some at McFaddendales Hardware store. Packs of a hundred for like $3.00. I have a ton of them still - have sent many out to people in need - for free of course. Quite honestly, I'm so sick of shipping stuff right now I'll still do it - but gees I sold a lot of stuff and put in a ton of Post Office time lol.
Moral of the story - Don't go overboard on spare parts ! Your rifle isn't prone to breakage.