Is Divi Booster a recommended plugin for Wordpress?

Yes. https://docs.woothemes.com/document/configuring-woocommerce-settings/

Well first off I wouldn't recommend trying to power your client's database with the standard WordPress database instance. They can use WordPress for the website all they like. But with millions of records like that, you really don't want to move those data in with WordPress. For that matter, you're probably gonna want a dedicated database server to answer those questions, if the client doesn't already have one. You might also need to investigate something like Elastic Search, depending on what your search requirements are and what sort of database they are using (pay close attention to whether or not it provides full text search ability). Beyond that, and somewhat obviously, you're also gonna want SSL on the website and to make sure you're up to date on federal and state privacy requirements for data like this, as well as best practices for securely transferring data over the web (port configuration, ssh keys, etc). The privacy concerns would raise a flag for me. If those data have anything personally identifiable in them - which they very probably do - then you're really gonna want to make sure you do your research on security of personally identifiable information. Government specific privacy /security /auditability requirements may also apply, so double check that too.

I've been down this road before (two custom CMS platforms built over the years), and to play devil's advocate here: do you really need to build a custom CMS? What problem are you up against that aren't being solved by existing solutions? If those problems aren't fundamental, how much time and money will be saved by creating plugins for an existing CMS? I know the temptation to start from scratch, but if this is something to be used in production, the todo list is staggering: you'll need user authentication, security on the front and back end, admin interfaces, plugin infrastructure for extensibility, theming (if this will be used for multiple clients), third party integrations — and that's all before you get into really annoying and notoriously finicky shit like media uploads and i18n. If it's for learning or a hobby, go nuts, but if you're planning to use this with clients, save yourself the heartache and late nights and use something established and open source. Join the community and help fix existing solutions' shortcomings — that's a bigger overall contribution to the world. Good luck!

Take the # of hours it takes you to do it and charge $50/hour. That's the price. Eventually you can charge $100/hour but that will require a bigger customer. If the customer is small < $1M in gross sales per year - charge $50/hour If the customer id medium < $1-5M / sales - charge $75 Over $5M - charge $100 The challenge you'll face is clearly defining the expectations and handoff so that you're not stuck doing stuff that you can charge for and always getting interrupted from past customers.

There is a lot of points in your question which unless someone has the exact same setup as you - I don't think you will get the answers you are looking for. I believe you should limit it by asking yourself: What is the ONE thing the MLM plugin I am looking for can do so that it achieves the maximum amount of result to my business by using it? IMHO - it would be the ability to set commissions for the products you sell and award commissions to those affiliates that actually earn you sales. All other features, while nice, are ancillary features that from the perspective of the affiliate - don't really need in order to be successful. In that regard - I would suggest https://affiliatewp.com/. When it comes to running affiliate programs on WP sites, it wins out as the best in my experience. Hope this helps! Glad to assist any further if you need help.