Audio, almost always. Viewers tolerate mediocre video far more than they tolerate hard-to-hear or noisy audio, so it gives the biggest perceived quality boost per pound spent.
Yes, for now. Advanced gear without the skills to use it well rarely improves content quality and often adds complexity that slows you down.
When your current equipment is consistently limiting your content quality or workflow speed, not just because newer gear exists. Consistency matters more than gear at every level.