A build vs. buy decision is always a hotly contested one amongst different stakeholders. More so, in early stage start-ups with comparatively lower product runway, uncertainty about delivery a compelling value proposition and challenges of distribution.
Having gone through these a couple of times and consulting a few other 0-1 startups on their technology + product, I would suggest looking at a Build vs. Buy decision from answering these 4 key questions:
- Is the missing feature/component of strategic value?
- Is the missing capability a true differentiator, core to the main value proposition or company's product strategy?
- Is the missing capability a minimum market requirement, something that is needed for the customer to consider your product in the category?
2. Do you have in-house technical expertise ?
- Do you have existing in-house  technical expertise vs. needing to hire contractors/freelancers to get this done?
- If you have existing in-house technical expertise, can this new capability be managed along with handling the existing backlog of priorities?
- Does this new feature/capability need extensive maintenance and updates, beyond the initial development?
3. Is it cost-effective?
- Have we factored in the maintenance and update costs for both the scenarios?
- Are the capabilities complex? As in, is there any external impact such as heavy regulation, integration complexities or needs a high volume of data?
4. Is the vendor reliable?
- If the vendor themselves are a startup, will they sustain / achieve PMF?
- Will they be able to meed your "needs"?
This is assuming the founding team / founder responsible for the decision has differentiated the "needs" from a "nice-to-have"