Configuring XBees is easy using the X-CTU application from digi.com, the manufactures of the XBee modules.
The only real issue of choosing the right XBee modules for the application. To have two Arduinos communicate with each other, Series 1 radios are the best choice. They come, then in regular and Pro versions. The Pro versions have longer range, but consume more power. Once the version is selected, there is still the choice of antenna type. They come with 4 different kinds of antennas. The choice of antenna affects range.
Attaching the XBee to the shield, and attaching the shield to the Arduino is trivial.
I am wondering if its just easier to use 2 or 3 pcs and connect arduinos via usb instead.
You can't do that. The USB port on the Arduino allows the Arduino to act like a USB slave. The PC that is at the other end of the USB cable is the USB master.
Two slaves can not talk to each other, and one Arduino can not be reconfigured as a USB master.
You can use the serial port on each to communicate via a wire, but that does not involve the USB port.
Choosing appropriate switches will probably need to be done on a case-by-case basis. The weight of the object, the placement of the switch, and the sensitivity of the switch are things to consider. Also whether you can actually acquire the switch.
In general, switches are the least expensive part of a project. It's not worth trying to shave 3 cents of the price of 20 switches. Find something locally available that you can hold and evaluate before you buy.
Switches report either off or on. All switches. The Arduino won't care what type of switch is connected to it.
No direct experience with IR emitters, but there are plenty of posts on the forum about them.