When is the ‘Right’ Time to engage a professional … ?

I’m an aspiring first time contractor of a yet to be designed home on the coast somewhere. I won’t be the builder but perhaps I can be actively involved as the General Contractor. I’ve read much of the material here and at other helpful sites. I’ve amassed interesting floor plans and designs but I don’t have the in-depth knowledge of many experienced people here.

So, when is the right time to seek and engage an Architect, builder, or designer? And, in which order?
What are the prerequisities that make that a fruitful and efficient process for all parties.

I don’t have a specific lot purchased. Is that a prerequisite?
I don’t have a specific timeline as it’s gated by life decisions and sale of primary home. But I’d like to identify and set a plan forward.

When is the right time?