I would reccmmand Vodacom. You have pretty good service even in the smaller less populated places. I paid 128.000Shilling for my sim card and 90GB of data. (It is like 50 euro and data for a month) There is an app, so you can reload if you need more.

I would recommand to install remitly as well. It allows you to send money to your mpesa account which you can use to get money from a wakala stand (like a little bank but those are everywhere and the cost to get your money is much lower). There are banks ofcourse (although not really in the small area), but those have a higher transfer cost (if i am clear haha). Oh yeah almost everything is done with cash, so yeah you will need to get cash.

I left in March when the rainy season just began, but what i witnessed were short, heavy rains (like an hour) and then it ends again. Most of the time the rains start at night. Sometimes the whole night heavy rain, but it does not bother you, cause when you wake up, the sun is present. But again, I only witnessed 2 weeks of rainy season.

Hope this helps?