Welcome to the community, @jajinga!
I’m not 100% sure that I understand your question, but I think that you’re looking to fully automate some sort of an eSignature system with Airtable, but without the help of external services that cost additional money.
Unfortunately, at this time, that would not be possible. You would be required to use external platforms if you want to automate an eSignature system.
However, many of these platforms can be very low cost or free to use.
For example, Make is free for up to 2 automations and up to 1,000 operations per month, and you can automate Make to work with any eSignature app.
And while SignNow is relatively cheap, eSignatures.io is likely even cheaper at only 49 cents per contract.
Hope this helps! 
p.s. I don’t offer free introductory calls, but feel free to contact me through my website and I can share my pricing structure with you: ScottWorld, Expert Airtable Consultant