You can start buying links to a new site immediately if you are convinced that the risk of losing search traffic does not harm the business.
The truth is that at the initial stage it is necessary to focus efforts on creating an excellent product that the client will like, regardless of whether you are on Google or not. That's what matters the most. It doesn’t matter what position you take in the search — the first or fourth, if no one wants to buy your product.
There is no good reason to invest in the purchase of eternal links if you do not have a well-optimized site and content that converts a site visitor into a client.