Question: which should be the "weakest link": Lipo - ESC - Motor ?

[AcidDrink]

If you really have to choose, lipos. In worst case you will kill them very quick. Esc should be the best.
Fantastic technical explanation above.
Note, this is going to matter more when you fly really hard, for light sport flying most decent lipos will be fine.
Tbh in case of 300x I'd be more concerned if drive train is appropriate for a very powerful motor