Should I build a custom Shopify app or find one on the App Store?

Default to the App Store. If an existing app covers 80%+ of what you need, it's far cheaper and maintained for you. Subscriptions add up, but so does custom development.

Build custom when no app does the core thing you need, you're stacking 4–5 apps to fake one workflow (slow and fragile), the data is sensitive enough that you don't want a third party touching it, or the feature is a genuine competitive edge. A private (custom) app installs only on your store and skips App Store review, so it can ship quickly. One rule: every app you install has a cost beyond its price — three well-chosen apps beat ten mediocre ones.