Visa for Filipinos and Other Survival Tips (Phnom Penh)

I'll be staying in Cambodia for two months for a volunteer programme and will be assigned to one of the centers in the province. I am excited about this opportunity but I do have a few concerns.

1. The cost of living in Cambodia. Would 5 million Cambodian Riel be enough for two months? Food and lodging will be provided but I have to shoulder other expenses.
2. A friend said they prefer to receive payment in dollars. How true is that?
3. Visa. Filipinos can stay in Cambodia for 21 days. Now what do I do on the 22nd? Can I apply for a visa there or should I get one in my country? Am I allowed to go to Laos for the day and go back on the same day or the next?
3. Is there a high risk of malaria especially in the provinces?

Thanks for the help!