Ask HN: Hire or build on my own, when starting out?

I am a developer, I run a profitable agency which is in the growing stage.

I had a SaaS idea one month ago, launched a waitlist, and got a lot of enterprise clients (Including very very famous ones) to sign up, I think I've found a product market fit of some sort without even building the product.

Currently, I don't have the time to build this product, nor do I have "a lot" of money.

Should I hire another engineer part-time to build this out in my supervision or should I slowly build it myself?

I am very cautious about the quality of this product as it will handle some important parts of the workload of our customers.