Sounds like you've already checked the gas key screw staking, so you don't need a whole BCG. For the carbine classes, you might consider picking up a Bravo Company bolt for under $70, which takes care of bolt and extractor concerns, and you can keep the Bushy bolt as a spare. If you're running a carbine-length gas system, make sure you get the bolt with the correct extractor setup. Upgrading the buffer might be a good idea, but if the rifle is running 100% as-is, that's not as critical as the gas key staking and the extractor are.
If you're paying $500 or $1000 and taking off work to go shoot a carbine class, and your rifle goes down halfway through causing that money to be wasted, that's a whole lot more expensive than having a spare bolt.