Become a CO-founder for a silicon valley based startup as a foreigner

The startup I work for is based in Silicon Valley, I am Swedish but are going to work in Korea where they have the most of their employees. I got an offer to become a CO-founder by owning 5% of the company valued at 1mil USD and have a lower salary. I want to take the offer but I feel like I don't have control over all laws, taxes etc. I have 3 documents in front of me:

1. Restricted stock purchase agreement
2. At-will employment confidential information, invention assignment, and arbitration agreement.
3. And the basic contract

To a total of 90 pages. I find it really hard to read and I don't trust myself reading this kind of documents.
In the most usually case, do I have any extra responsibilities by owning some of the stock in the company or is it basically like being a regular employee? Do I need to pay some extra tax for my shares or anything similar?

The company has it's majority of their employees abroad and only a few in SV. I don't have a US visa but I will get a working visa for Korea where I will work from.

How can I get help regarding this? Thank you.