Essential? None. Nice to have? Springs are wear items, so a complete spring kit, then perhaps extractor / ejector.
I think I've owned about a dozen various Polytech & Norinco M305/M14S over the past 20 years, so a wide variety of production dates and quality levels. To my recollection I've never had a part break on any of them. Less than perfect quality (rear sights & Chu wood stocks), yes. Replaced to upgrade (rear sights, stocks, trigger groups on a few) yes. But replaced due to breakage, no.