1. I do not know of a WiFi mouse/keyboard. Perhaps you mean RF (Radio Frequency). The difference:
BlueTooth (BT) is an RF device from a transceiver standpoint, but implements a standards-based protocol such that many device types can communicate across it. A BT dongle plugged into a PC requires drivers. An RF dongle for kb/mouse is typically proprietary; that is, only to be used for wireless kb/mouse communication, and also typically driverless. The RF set appears as a standard kb/mouse to the PC. If *ALL* you want to do is wireless kb/mouse, I'd go with RF from a reputable manufacturer, such as Logitech's unified receiver.
2. Don't buy the cheapest. Reviews matter here. BT is a protocol that's easy to get wrong, or just barely right.
3. One BT adapter will pair with multiple devices.