Visa, Yellow Fever vacc, Housing, Volunteering (Nairobi)

Greetings InterNations family! I am a fairly new member, making her first post. I seek advice/answers to the following questions, as I plan to travel to Kenya very soon.

Should I seek a visa to Kenya before arriving or can I wait to get processed at the airport, with some certainty of admittance? (I ask because the server is down @ the Kenyan consulate in my city & my online visa application is not getting processed for this reason.)

Should I get vaccinated for Yellow Fever? (I am getting conflicting answers on this one. I'm especially fighting this because I understand it weakens the liver.)

How much is the average rent for a studio OR shared apartment (1 bedroom) in the Westlands area of Nairobi?

Do you know of NGOs (small or large) seeking volunteers to teach English, self-defense or aerobics? (These are my specialties and I would be most happy to help out where needed.)

Thank you in advance for any and all responses. ~Jessica :)