Answer: 1.) You can use a USB WiFi adapter, install a dedicated PCIe WiFi card, or upgrade to a new motherboard with built-in WiFi. (That's three solutions but just want to give you flexibility with your problem.)
Answer: 2.) None of the processors would fit because they all have different series sockets. (The i5 processors have LGA1151 sockets and the i3 has an LGA1156 socket.)
Explanation: 1.) Usb WiFi adapter, dedicated PCIe WiFi card, OR a new motherboard with built-in WiFi
2.) None of the processors work