Hi, i already did such a plugin, but it was before the existence of CPT , so this would need code rewriting, but the basis is very good, let me know if you want more information/help to do your plugin.
i’m also working on a similar free project called WPbillings
You might also want to look into the WP E-Signature plugin by ApproveMe. They focus 100% on legally binding contracts, estimates, and proposals (UETA & ESIGN compliant) if you’re looking to collect signatures on court recognized contracts.
As far as I can tell WP eSignature is the most customizable (and secure) document singing application out there for WordPress.