Visa and Work Permit for Volunteering

Dear Sir or Madam,

I write to you bacause I have questions concerning immigration and work permit laws in Greece. I am considering an unpaid volunteer position for 6-9 months in Thermopylae, Greece for a Dutch non-profit organization that works on education for refugees. I am Vietnamese. My questions are as follows:

1. When filling in the Schengen Visa Application form, what is the most appropriate reason for my visit? "Work", "Cultural", or "Other: (Volunteer)"?

2. Because I would like to volunteer for 6-9 months, should I get a work permit to extend the duration of my stay, since I heard visas usually expire after 90 days? If so, what type of work permit should I apply for, and how much will it cost? Is it Type C?

3. This question is, I guess, a bit normative. I am also considering assuming the role of program manager after 4 months volunteering. As a program manager, I would have the option to be paid. Therefore:
a. Does the Greek government generally prefer/trust someone like me to be paid ot not paid when staying in Greece?
b. Is it wiser to state my intention to take on a paid job (a) when I apply for the visa; (b) when in Greece, just before I make the switch from volunteering to working; or (c) volunteering the whole time and not get paid at all? I can choose to not be paid and I will do so if it complicates the matter.

Best regards,
Chuong Do