I am an ERP specialist with a degree in Supply Chain management and over 25 years experience in ERP systems. I have developed and managed ERP systems since 1986 and have worked with over 7 different types. After reading your doc file I realize you have done most of the design work, creating a map of how you want your system to perform. Before we start building he system, we have to define the databases to be used. As for database sharing, it is better to maintain separate files in the same database for different companies, this way you won't mix data but you will maintain it all in one place. Whilst the products and prices, suppliers and POS schemes might vary from site to site the syntax and nomenclature must be singular so everyone is understood. After receiving the bid, I would begin by designing the infrastructure and then transferring the old data into the new.
I agree we must approach this from a project management stance, set specific tasks, give them starting dates and milestones.
Just a quick note, ERP Next is a cloud based ERP system, the core is identical to every ERP system. Based on your doc file the whole project is around 600 hours (3 months) with an 8% divergence. I expect it would be less, maybe 500 hours, but it's better to take plan longer and finish shorter than vice versa.