What is the best Wordpress theme to use to showcase many events happening, globally by country, category, city and date?

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

If you are going to use Social Media Marketing. I would recommend you create a Pinterest wall of your work and back link them to your site. Try that.

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.

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.

I think BuiltWith will do this for you. Plug in the domain and it will tell you which technologies it is built on. Most of the well-known video sites are custom jobs, but I'm sure WP could do what you want.