A few things to consider:
- If the money is not in the bank, then the company has not raised $100M. Until the money is wired, it can fall through.
- Without a liquidity event you cannot make any money from your equity. It doesn't matter how much the company is valued, unless someone buys the company or it goes public you will not likely be able to sell your shares.
- Most options come with a cliff, meaning you lose it all if you leave before 4 years. In this industry, 4 years is a long time. Be cognizant of the many ways in which you can lose your stock (cliffs, dilution, liquidity preferences, etc.)
- I'd double check with a lawyer, but if you earn equity in the US you must pay taxes to the US - it doesn't matter what country you live in.
- $400K invested wisely is enough for you to secure retirement, and then focus on risking everything for your own startup where you own 100% of the equity and upside.
Personally, I would take the money, secure retirement ASAP, and then take your own shot at a startup.