I'd argue that in a startup, at a very early stage, you can use external services even for core functions, if they work well enough.

Look at Intercom: now it's a juggernaut, but it started as an API mashup with a UI in front of it. If you can hit the market and start getting revenue faster you should. Many services have pay as you go and/or startup discounts.

That's your launch platform, then you start working from there.