Build a Web Application as SAAS

So, basically, I am intend to have a SAAS software down in Brazil, with the following architeture: Backend Python Postgres Security Front end HTML CSS Angular API RESTFULL Right now, I have some entities requirements designed. They are mainly CRUD tables, where most of them are CODE/Description entities (Product Type, Product Group, Product, Customer, Supplier, etc), since I did not have yet the transaction tables documented (Account receivables, Payment Accounts, Goods Movement, Sales, etc) The system has to be based on modules, which means a Module has a set of system functions. System is based on Contract, which each new sign-up create a Stronger Key to all owner user data. (All database tables will have the contract_id key to guarantee user info safe) Country, State and City are common tables to all system, so there is no contract_id on those. We can have more tables like these. Each contract on client side can have multiple companies and invited users, since a company can have the headquarter and some branches, plus managed users withou new sign-up. Transactions functions should considerer the company_id on tables. A CSS template will be provide to you if we go ahead with the deal, and it has to be used in the all system based on Angular framework, HTML, CSS and API RESTFUL. template: [login to view URL] API should be designed as much as possible to attendant web and mobile applications, so it is better to work on the logic between Angular and API. The CSS has to keep the reponsivity if something has to be changed to attendant the SW. Since it is a SAAS software, it is necessary to be able to integrate later with some payment methods, such as Credit card, Pay Pal and others. On the backoffice side, I would like to have the management tools to follow all new contracts, expiration ones, usability, etc. ________________________ 1- Admin (Back office) 2- Supplier/vendor board 3- Registered user board